2G 



OPTICS. 



Fig. 30. 



violet, will occupy larger spaces, or will 

 be more expanded. Hence the coloured 

 spaces have not the same ratio to each 

 other as the lengths of the spectrum ; 

 and therefore this property is called the 

 irrationality of dispersion, or of the 

 coloured spaces in the spectrum. This 

 property is distinctly shewn in the above 

 figure, from which it will also appear, 

 that the mean ray, m n, is among the 

 blue rays in the oil of cassia spectrum ; 

 and among the green rays in the sul- 

 phuric acid spectrum. 



As the examination and measurement 

 of this property of transparent bodies is 

 very difficult, we can give only a list "of 

 substances, arranged nearly in the order 

 into which they contract the less refran- 

 gible spaces, and expand the more re- 

 frangible ones, according to the experi- 

 ments of Dr. Brewster. 



CHAPTER X. Imperfections of the 

 Refracting Telescope Achromatic 

 Telescope Dr. Blair's Aplanatic 

 Telescopes. 



THE application of the principles ex- 

 plained in the preceding chapter to the 

 improvement of the refracting telescope, 

 forms one of the most interesting por- 

 tions of optical science. If we take a 

 prism A b C, Jig. 29, having the angle 

 b A C = A C B, and formed of the very 

 same substance as A C B ; and if we 

 place it in contact with ACB, it will 

 destroy or correct the spectrum P T, by 

 refracting the whole of the rays se- 

 parated by the prism A B C to the same 

 point Y, where, by their mixture, they 

 will form white light, the separation 

 arising from the refraction and disper- 

 sion of the first prism being exactly com- 

 pensated or balanced by the refraction of 

 the second prism*. If the angle b A C 

 is less than ACB, the second prism will 

 neither correct the refraction nor the 

 dispersion of the first, and a short spec- 

 trum will be formed a little above Y ; 

 and if b A C is greater than ACB, the 

 second prism will more than correct the 

 first, and a short spectrum will be 

 formed below Y. Hence it is manifest, 

 that by combining prisms of the same 

 kind of glass in the manner shown in 

 Jig. 29, we cannot make the ray S Y 

 deviate from its direction without pro- 

 ducing the prismatic spectrum ; that 

 is, we cannot produce refraction without 

 producing colour. Now, as we have 

 already shown that every lens acts ex- 

 actly like a prism, it follows also, that 

 we cannot combine a concave and a 

 convex lens of the same glass to refract 

 in opposite directions, without produ- 

 cing colour : for when we succeed in 

 finding two which destroy the colours, 

 we shall find also that the refraction is 

 destroyed, and that the two lenses put 

 together resemble a watch-glass, which 

 has not the property of a lens at all. 



In the same manner a.s we have now 

 reasoned respecting prisms or lenses of 

 the same glass, Sir Isaac Newton rea- 

 soned respecting prisms and lenses of all 

 kinds of glass and of all bodies whether 

 solid or fluid, because he believed that all 

 bodies whatever had the same dispersive 

 power, or produced the same length of 

 spectrum in proportion to their mean 

 refraction. Had this been true, Sir 



* This is evident also from the consideration that 

 the faces A 6, B C are parallel, and that the two 

 prisms are nothing more than a plane lens like that 

 shovvu at B, in Jig. 5. 



