184 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



of all the colors, as black is the want or deprivation of 

 color ; which may be proved by this instrument. 



Though seven different colors are distinguishable in the 

 prismatic spectrum, yet, upon examining the matter with 

 more accuracy, we shall see that there are, in fact, only 

 three original colors red, blue, and yellow, for the orange 

 being situated between the red and yellow, is only the mix- 

 ture of these two ; the green, in like manner, arises from 

 blending the blue and yellow, and the violet results from 

 the blue and red. Price, $1.00. 



Glass Box for Experiments on Refraction. (Fig. 700.) 

 This consists of a metallic box, open at top, from 8 to 14 

 inches long, and from 5 to 9 inches wide, having true plate- 

 glass sides. Each end has a circular aperture, about 2 inches 

 wide, the one having a meniscus glass with the convex sur- 

 face outwards, and the other having one with the concave 

 surface outwards. The box is made water-tight, and is usu- 

 ally supported on a neat base, and furnished with a cock, 

 for the more easily emptying it of water. 



Fig. 700. 



To use it, cause a ray, or pencil of light, from a hole in 

 the window, to fall on the bottom of the box, where you 

 may make a mark ; then fill the box with water, without 

 moving it out of its plaee, and you will see that the pencil 

 of light, instead of falling on the mark, as before, will be 

 refracted, or bent out of its former course. A few drops of 

 milk added to the water will make the course of the ray of 

 light more distinct. If a small mirror be laid on the bottom 

 of the box when filled with water, the light will be reflected 

 from it, and will be observed to suffer the same refraction as 



