"A prism is a transparent body with two refraction surfaces that lie in intersecting planes. The angle formed by these planes is called the refracting angle." -Avery 1895

Prism

"A prism is a transparent body with two refraction surfaces that lie in intersecting planes. The angle…

A profile illustration of two women.

Profile of Two Women

A profile illustration of two women.

Two rabbits.

Two Rabbits

Two rabbits.

"...represents two parabolic reflectors, mn and op. It is a peculiarity of such reflectors that rays starting from the focus, as F will be reflected as parallel rays, and that parallel rays falling upon sucha reflector will converge at the focus, as F'. Hence, two such reflectors may be placed in such a sposition that sound waves starting from one focus shall, after two reflections, be converged at the other focus...Two reflectors so placed are said to be conjugate to each other." -Avery 1895

Parabolic Reflectors

"...represents two parabolic reflectors, mn and op. It is a peculiarity of such reflectors that rays…

Two roses.

Two Roses

Two roses.

Bilingual Counting Card featuring illustrations of two Sailfishes.

Bilingual Sailfish Counting Card 2

Bilingual Counting Card featuring illustrations of two Sailfishes.

English Counting Card featuring illustrations of two Sailfishes.

English Sailfish Counting Card 2

English Counting Card featuring illustrations of two Sailfishes.

Spanish Counting Card featuring illustrations of two Sailfishes.

Spanish Sailfish Counting Card 2

Spanish Counting Card featuring illustrations of two Sailfishes.

Bilingual Counting Card featuring illustrations of two Sandcastles.

Bilingual Sandcastle Counting Card 2

Bilingual Counting Card featuring illustrations of two Sandcastles.

English Counting Card featuring illustrations of two Sandcastles.

English Sandcastle Counting Card 2

English Counting Card featuring illustrations of two Sandcastles.

Spanish Counting Card featuring illustrations of two Sandcastles.

Spanish Sandcastle Counting Card 2

Spanish Counting Card featuring illustrations of two Sandcastles.

An orthographic illustration of a die displaying sides 1, 2, and 4.

Die with sides 01, 02, 04

An orthographic illustration of a die displaying sides 1, 2, and 4.

An orthographic illustration of a die displaying sides 1, 3, and 2.

Die with sides 01, 03, 02

An orthographic illustration of a die displaying sides 1, 3, and 2.

An orthographic illustration of a die displaying sides 2, 1, and 3.

Die with sides 02, 01, 03

An orthographic illustration of a die displaying sides 2, 1, and 3.

An orthographic illustration of a die displaying sides 2, 3, and 6.

Die with sides 02, 03, 06

An orthographic illustration of a die displaying sides 2, 3, and 6.

An orthographic illustration of a die displaying sides 2, 4, and 1.

Die with sides 02, 04, 01

An orthographic illustration of a die displaying sides 2, 4, and 1.

An orthographic illustration of a die displaying sides 2, 6, and 4.

Die with sides 02, 06, 04

An orthographic illustration of a die displaying sides 2, 6, and 4.

An orthographic illustration of a die displaying sides 3, 2, and 1.

Die with sides 03, 02, 01

An orthographic illustration of a die displaying sides 3, 2, and 1.

An orthographic illustration of a die displaying sides 3, 6, and 2.

Die with sides 03, 06, 02

An orthographic illustration of a die displaying sides 3, 6, and 2.

An orthographic illustration of a die displaying sides 4, 1, and 2.

Die with sides 04, 01, 02

An orthographic illustration of a die displaying sides 4, 1, and 2.

An orthographic illustration of a die displaying sides 4, 2, and 6.

Die with sides 04, 02, 06

An orthographic illustration of a die displaying sides 4, 2, and 6.

An orthographic illustration of a die displaying sides 6, 2, and 3.

Die with sides 06, 02, 03

An orthographic illustration of a die displaying sides 6, 2, and 3.

An orthographic illustration of a die displaying sides 6, 4, and 2.

Die with sides 06, 04, 02

An orthographic illustration of a die displaying sides 6, 4, and 2.

Signal for the letter B, or the number two.

Signal B

Signal for the letter B, or the number two.

"Two fixed pulleys used to raise W." -Comstock 1850

Simple Pulley

"Two fixed pulleys used to raise W." -Comstock 1850

"Partly fill two bottles with water. Connect them by a bent tube that fits closely into the mouth of one, and loosely into the mouth of the other. Place the bottle under the receiver and exhause the air. Note and record what takes place. Admit air to the receiver. Note and record what takes place." -Avery 1895

Bottle Siphon

"Partly fill two bottles with water. Connect them by a bent tube that fits closely into the mouth of…

"Skull of two-toed sloth." —The Encyclopedia Britannica, 1903

Sloth Skull

"Skull of two-toed sloth." —The Encyclopedia Britannica, 1903

Probability Spinner 2

Spinner

Probability Spinner 2

Bilingual Counting Card featuring illustrations of two Sponge Divers.

Bilingual Sponge Diver Counting Card 2

Bilingual Counting Card featuring illustrations of two Sponge Divers.

English Counting Card featuring illustrations of two Sponge Divers.

English Sponge Diver Counting Card 2

English Counting Card featuring illustrations of two Sponge Divers.

Spanish Counting Card featuring illustrations of two Sponge Divers.

Spanish Sponge Diver Counting Card 2

Spanish Counting Card featuring illustrations of two Sponge Divers.

Bilingual Counting Card featuring illustrations of two Starfishes.

Bilingual Starfish Counting Card 2

Bilingual Counting Card featuring illustrations of two Starfishes.

English Counting Card featuring illustrations of two Starfishes.

English Starfish Counting Card 2

English Counting Card featuring illustrations of two Starfishes.

Spanish Counting Card featuring illustrations of two Starfishes.

Spanish Starfish Counting Card 2

Spanish Counting Card featuring illustrations of two Starfishes.

"A two-pole front-connected knife switch fused a the handle end." —Croft 1920

Two-pole Knife Switch

"A two-pole front-connected knife switch fused a the handle end." —Croft 1920

"A simple telegraph consisting of two telephone receivers, two batteries, and two keys." -Avery 1895

Simple Telegraph

"A simple telegraph consisting of two telephone receivers, two batteries, and two keys." -Avery 1895

Dual Blank Lab Thermometer

Dual Lab Celsius Centigrade Thermometers

Dual Blank Lab Thermometer

Dual Lab Thermometers 0 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 0 Degrees Celsius Centigrade

Dual Lab Thermometers -5 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers -5 Degrees Celsius Centigrade

Dual Lab Thermometers -10 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers -10 Degrees Celsius Centigrade

Dual Lab Thermometers -15 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers -15 Degrees Celsius Centigrade

Dual Lab Thermometers -20 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers -20 Degrees Celsius Centigrade

Dual Lab Thermometers 5 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 5 Degrees Celsius Centigrade

Dual Lab Thermometers 10 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 10 Degrees Celsius Centigrade

Dual Lab Thermometers 15 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 15 Degrees Celsius Centigrade

Dual Lab Thermometers 20 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 20 Degrees Celsius Centigrade

Dual Lab Thermometers 25 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 25 Degrees Celsius Centigrade

Dual Lab Thermometers 30 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 30 Degrees Celsius Centigrade

Dual Lab Thermometers 35 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 35 Degrees Celsius Centigrade

Dual Lab Thermometers 40 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 40 Degrees Celsius Centigrade

Dual Lab Thermometers 45 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 45 Degrees Celsius Centigrade

Dual Lab Thermometers 50 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 50 Degrees Celsius Centigrade

Dual Lab Thermometers 55 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 55 Degrees Celsius Centigrade

Dual Lab Thermometers 60 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 60 Degrees Celsius Centigrade

Dual Lab Thermometers 65 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 65 Degrees Celsius Centigrade

Dual Lab Thermometers 70 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 70 Degrees Celsius Centigrade

Dual Lab Thermometers 75 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 75 Degrees Celsius Centigrade

Dual Lab Thermometers 80 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 80 Degrees Celsius Centigrade

Dual Lab Thermometers 85 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 85 Degrees Celsius Centigrade

Dual Lab Thermometers 90 Degrees Celsius Centigrade

Dual Lab Celsius Centigrade Thermometers

Dual Lab Thermometers 90 Degrees Celsius Centigrade