OF NEWTON'S OPTICS. 



29 



on which the perfection of refracting 

 telescopes has since been found to de- 

 pend, seems to have pressed itself for- 

 ward, and even to have courted his at- 

 tention in the very experiments from 

 which he deduced his erroneous conclu- 

 sion. (Fig. 32.) Let ABC, A'B'C' 

 Fig. 32. 



be two prisms, formed of different trans- 

 parent substances. Let S B, S' B' be 

 rays of the sun falling on them in pa- 

 rallel directions. Let B V, B'V be the 

 most refracted or violet light in each, 

 and B R, B'R' the least refracted or red 

 light. The deviation of the rays from 

 their original direction, produced by the 

 refraction of the prisms, will be different 

 for each component part of the incident 

 light. Newton supposed that the de- 

 viations of the different coloured lights 

 from the common direction when inci- 

 dent, have to each other a certain fixed 

 proportion ; so that with the same aver- 

 age refraction or deviation from their 

 common original direction, they would 

 be dilated or separated from each other 

 in the same degree. This may perhaps 

 be more easily comprehended if thus 

 explained. Let ABC, A' B' C' be two 

 prisms of different materials, receiving 

 parallel rays S B, S' B', of solar light. 

 Let the lines B M, B' M' divide the an- 

 gles V B R, V B' R', formed by the ex- 

 treme red and violet rays into equal parts, 

 or so that V B M shall be equal to 

 R B M, and also V B' M' to R' B' M'. 

 Also, suppose the prisms to have such 

 refracting angles, that the rays B M, 

 B' M' shall be parallel. The deviations 

 of these rays, s B M, s' B' M', from their 

 original directions B s, B'*' must be 

 equal. Under these circumstances New- 

 ton concluded that the angles V B R 

 and V'B'R' would be equal, and that 

 the deviation of every ray in the spec- 



trum VR, from its original direction B s, 

 would be equal to the deviation of the 

 similar ray in the spectrum V R' from 

 its original direction B'*'. Such is not 

 the fact, and it is almost inconceivable 

 how Newton, who had avowedly ex- 

 amined the spectra produced, not only 

 by prisms of different kinds of glass, but 

 also by liquids contained in hollow glass 

 prisms, could have escaped noticing a 

 fact that would at once have led him to 

 the discovery of achromatic telescopes. 



In fact the prisms being circumstanced 

 as we have just described, so as to pro- 

 duce equal deflections of the sunbeam 

 from its original direction, the dilatation 

 or dispersion of the rays from each other, 

 and which may be measured by the di- 

 vergence V B R, V B' R' of the extreme 

 rays, will be different according to the 

 material of which the prism is composed. 

 Newton, on the other hand, concluded 

 that when the deflection of the sun's 

 beam by different prisms was the same, 

 the dispersion would also be the same. 

 Had he thought of measuring the lengths 

 of spectra produced by different prisms, 

 equally deflecting the light, he could not 

 have tailed to have found them different, 

 and would have naturally been led to the 

 discovery of achromatic telescopes, as 

 we shall now explain. 



Since prisms of different materials, 

 with an equal deflection of the beam, 

 produce spectra of different lengths, and 

 since also the length of a spectrum va- 

 ries with the position of the prism, or, 

 what is the same, with the deflection of 

 the light, it follows that if two prisms of 

 different materials be exposed to beams 

 of the sun's light, one of them may be 

 turned until such a position be given to 

 it, that the length of the spectrum pro- 

 duced by it shall be equal to the length 

 of the spectrum produced by the other 

 prism. 



In this case the deflection of the beam 

 by the two prisms producing equal 

 spectra must be different, for if not, as 

 we have before stated, the spectra would 

 have different lengths. If one of the 

 prisms be inverted with respect to the 

 other, all other things remaining the 

 same, the spectra will still keep the same 

 length, but the colours will be reversed. 

 Now suppose that instead of transmit- 

 ting different beams of light through the 

 two prisms, the same beam be succes- 

 sively transmitted through them, the one 

 being placed behind the other, but the 

 former arrangement being in all other 

 respects preserved, it is quite evident 



