OPTICS. 



Ms hand is in the centre of concavity ; 

 mid he will imagine he may shake hands 

 with his image. If he reach his hand fur- 

 ther, the hand of the image will pass by 

 his hand, and come between it and his 

 body ; and if he move his hand towards 

 cither side, the hand of the image will 

 move towards the other ; so that what- 

 ever way the object moves, the image 

 will move the contrary way. A by-stand- 

 erwill see nothing of the image, because 

 none of the reflected rays that form it 

 enter his eyes. 



The images formed by convex specula 

 are in positions similar to those of their 

 objects ; and those also formed by con- 

 cave specula, when the object is between 

 the surface and the principal focus : in 

 these cases the image is only imaginary, 

 as the reflected rays never come to the 

 foci from whence they seem to diverge. 

 In all other cases of reflection from con- 

 cave specula, the images are in positions 

 contrary to those of their objects, and 

 these images are real, for the rays after re- 

 flection do cv,me to their respective foci. 

 These things are evident from what has 

 gone before. See Miuwon. 



" Of colours and the different refrangi- 

 bility of light." The origin of colours is 

 owing to the composition which takes 

 place in the rays of light, each hetero- 

 geneous ray consisting of innumerable 

 ravs of different colours; this is evident 

 from the separation that ensues in the 

 well-known experiment of the prism. A 

 ray being let into a darkened room (fig. 

 11) through a small round aperture, z, and 

 falling on a triangular glass prism, x, is by 

 the refraction of the prism considerably 

 dilated, and will exhibit on the opposite 

 wall an oblong image, a b, called a spec- 

 trum, variously coloured, the extremities 

 of which are bounded by semicircles, and 

 the sides are rectilinear. The colours 

 are commonly divided into seven, which, 

 however, have various shades, gradually 

 intermixing at their juncture. Their or- 

 der, beginning from the side of the re' 

 fracting angle of the prism, is red, orange, 

 yellow, green, blue, purple, violet. The 

 obvious conclusion from this experiment 

 is, that the several component parts of 

 solar light have different degrees ofre- 

 frangibility, and that each subsequent ray 

 in the order above mentioned is more re- 

 frangible than the preceding. 



As a circular image would be depicted 

 by the solar ray unrefractedby the prism, 

 so each ray that suffers no dilatation by 

 the prism would mark out a circular im- 

 age, y. Hence it appears, that the spec- * 



trum is composed of innumerable circles 

 of different colours. The mixture, there- 

 fore, is proportionable to the number of 

 circles mixed together (fig. 12) ; bat all 

 such circles are mixed together, whose 

 centres lie between those of two contin- 

 gent circles, consequently the mixture is 

 proportionable to the interval of those 

 centres, i. e. to the breadth of the spec- 

 trum. If therefore the breadth can be 

 diminished, retaining the length of the 

 rectilinear sides, the mixture will be 

 lessened proportionality, and this is done 

 by the following process. 



At a considerable distance from the 

 hole, z, place a double convex lens, A B 

 (fig 13), whose focal length is equal to 

 half that distance, and place the prism oc, 

 behind the lens ; at a distance behind the 

 lens, equal to the distance of the lens 

 from the hole, will be formed a spectrum, 

 the length of whose rectilinear sides is 

 the same as before, but its breadth much 

 less; for the undiminished breadth was 

 equal to a line subtending, at the distance 

 of the spectrum from the hole, an angle 

 equal to the apparent diamater of the 

 sun, together with a line equal to the 

 diameter of the hole ; but the reduced 

 breadth is equal to the diameter of the 

 hole only ; the image of the hole formed 

 by the lens at the distance of double its 

 focal length, is equal to the hole ; there- 

 fore, its several images in the different 

 kinds of rays are equal to the same, i. e. 

 the breadth of the reduced spectrum is 

 equal to the diameter of the hole. 



A prism ABC, (fig. 14, Plate II.) 

 placed in an horizontal position, would 

 project the ray into an oblong form, as 

 has ben seen ; apply another horizontal 

 prism, A D B, similar to the former, to 

 receive the refracted light emerging from 

 the first, and having its refracting angle 

 turned the contrary way from that of the 

 former. The light, after passing through 

 both prisms, will assume a circular form, 

 as if it had not been at all refracted. 



If the light emerging from the first 

 prism be received by a second, whose 

 axis is perpendicular to that of the for- 

 mer, it will be refracted by this trans- 

 verse prism into a position inclined to the 

 former, the red extremity being least, 

 and the violet most removed from its for- 

 mer position ; but it will not be at all al- 

 tered in breadth. 



Close to the prism A (fig. 15), place a 

 perforated board, a b, and let the refract- 

 ed light (having passed through the small 

 hole) be received on a second board, c r/, 

 parallel to the first, and perforated in like 



