OF NEWTON'S OPTICS. 



the base of the prism. From the eye, let 

 lines he supposed to be drawn to the base, 

 nclined to it at that angle which limits 

 the reflexion of the violet li^ht. These 

 lines being all equally inclined to the base, 

 must meet it at points which lie in the 

 arc of a circle. Let this arch be V V, 

 fig. 40. Again, let lines be drawn at the 



FigAQ . 



1, r. 



limiting angle of the indigo rays. This 

 angle being less than that for the violet, 

 the corresponding arc 1 1' will be beyond 

 V V. In the same manner the limiting 

 arcs B B', GG', Y Y', O O', RR', cor- 

 responding to the other prismatic lights, 

 blue, green, &c. may be drawn. 



It follows then, that all the violet rays 

 in the solar light will be reflected from 

 the part of the base of the prism whose 

 boundary is VV'GI; all the indigo 

 from 1 1' G I ; all the blue from B B' G I ; 

 all the green from GG'GI; all the 

 yellow from YY'GI; all the orange 

 from O O' G I, and all the red from 

 R R' G I. Hence it appears that the 

 space between the arcs, V V and 1 1', is 

 illuminated with a pure violet light only ; 

 that between 1 1' and B B' is illuminated 

 by both violet and indigo mixed; be- 

 tween B B' and G G' there is a mixture 

 'of violet, indigo, and blue; between 

 G G' and Y Y' is a mixture of the former 

 colours, with the addition of green ; from 

 Y Y' to O O', yellow is added to the com- 

 pound ; the next arched band introduces 

 orange, and the last the red. Now the 

 last mixture constitutes a pure white. 

 The former also a white, but one which, 

 being deprived of the pure red rays, takes 

 a faint tint approaching a bluish colour, 

 but which is not distinguishable from a 

 perfect white. In the next space the red 

 and orange being removed, the mixture 

 produces a greenish blue, which rapidly 

 deepens, and becomes a strong blue, 

 when the yellow rays are removed. The 

 arc towards its inner termination is a 

 pure violet. 



Newton next applies his theory to ex- 

 plain the phenomena of rainbows. As 



this subject has been already fully dis- 

 cussed in our treatises on Optics accord- 

 ing to the same principles, and in exactly 

 the same manner as it is treated by 

 Newton, it is not necessary to repeat it 

 here. 



CHAPTER VI. 



On the phenomena exhibited by thin 

 transparent plates the theory of the 

 Jits of easy reflexion and transmission 

 deduced from these phenomena. 



(50.) THE first book of Newton's Optics 

 contains the discussions which have 

 been detailed in the last four chapters. 

 In these investigations a ray of light 

 upon its impact on the surface of any 

 medium is considered to undergo one 

 of two effects, viz. either to pass into the 

 medium on which it impinges in a deter- 

 minate direction, in which direction it is 

 supposed to persevere through the entire 

 medium ; or to be reflected back from 

 the surface into the medium from whence 

 it came, following also and persevering 

 in a rectilinear course. We are now 

 about to accompany this great scrutineer 

 of nature through a more subtle analysis 

 of the process to which- a beam of light 

 is submitted when it encounters the sur- 

 face which separates two media of dif- 

 ferent densities. 



If it were possible to divide the me- 

 dium which a ray of light penetrates 

 into a series of plates, the thickness of 

 which should be minute to an extreme 

 degree, and to examine the state of the 

 ray during its transmission through 

 each of them, we should attain the end 

 which we desire. Although it would 

 perhaps be difficult to effect this very 

 minute subdivision by direct mechanical 

 means, yet numerous expedients present 

 themselves, and those too of a character 

 sufficiently familiar, by which the phe- 

 nomena in question may be brought 

 under examination. Indeed, these phe- 

 nomena were long the subjects of daily 

 observation, and may almost be said to 

 have been the sport and toy of children ; 

 but, like many other natural effects, not 

 less wonderful, which are continually 

 passing under our eyes, they had failed 

 to excite the attention or stimulate the 

 curiosity of those who, by faculties and 

 acquirements, were qualified to behold 

 in them manifestations of the laws and 

 principles on which the works of nature 

 are constructed. 



(51.) If a small quantity of soap be 

 mixed with water, the latter acquires a 



