30 



A POPULAR ACCOUNT 



that the dispersion of the one prism 

 will have a tendency to neutralise the 

 dispersion of the other, and that in the 

 beam emerging from the second prism, the 

 prismatic lights will be so mingled as to 

 render the emergent beam nearly colour- 

 less. This will appear from considering 

 that the tendency of the one prism to 

 disperse the rays in one way, bringing the 

 violet ray highest and the red lowest, is 

 exactly equal to the tendency of the 

 other to disperse the light in the oppo- 

 site way, bringing the violet lowest and 

 the red highest. But this mutual com- 

 pensation will not obtain in the deflection 

 of the light, since the power of the se- 

 cond prism to deflect downwards is not, 

 in its actual position, equal to the power 

 of the first to deflect upwards ; so that 

 the prism which has the less deflecting 

 power will destroy so much of the de- 

 flecting effect of the other as is equal to 

 its own, but an effective deflection will 

 remain, by which the beam will be 

 turned from its original direction. 



Thus we arrive at the important fact, 

 that a beam of light may have its direc- 

 tion changed by refraction, so that the 

 directions of all its component rays shall 

 be equally changed, or nearly so, al- 

 though they be differently refrangible. 

 What may be done by prisms may 

 also be effected by lenses ; and therefore 

 an object-glass of a telescope may be 

 so constructed as to collect all the 

 rays of different refrangibilities nearly 

 to the same focus,* and thus an achro- 

 matic telescope may be formed. Such 

 was the discovery that Newton left to 

 adorn a future age, a discovery pre- 

 sented to him by his own experiments, a 

 fact rendered not improbable by his 

 own reasoning, consistent with his own 

 theory, and soliciting investigation and 

 inquiry at almost every step of his own 

 researches, yet which investigation and 

 inquiry he seems, by an unaccountable 

 pertinacity, to have stepped out of his 

 way to avoid. 



Newton seems not to have maintained 

 an uniform opinion at all times on this 

 point. The first edition of his Optics was 

 published in 1704, and the second in 

 1717. In both of these he pronounces 

 the improvement of refracting telescopes 



* In strictness, two prisms or lenses will only bring 

 two colours accurately together, the law of disper- 

 sion being different throughout the whole spectrum: 

 the rest, however, will be very nearly coincident, and 

 consequently colour very nearly got rid of. By the 

 combination of three prisms or lenses, three colours 

 may be accurately combined, and the rest still more 

 nearly than before ; and so in succession. 



to be desperate. And yet, in a letter to 

 Mr. Oldenburg, dated July, 1672, three 

 years before his " discourse about light 

 was written at the desire of some gen- 

 tlemen of the Royal Society," he vindi- 

 cates himself from a charge of Dr. 

 Hooke, " who reprehended him for 

 laying aside the thoughts of improving 

 optics by refractions," in the following 

 words " What I said was in respect of 

 telescopes of the ordinary construction, 

 signifying that their improvement is not 

 to be expected from the well figuring of 

 glasses, as opticians have imagined. But 

 I despaired not of their improvement by 

 other constructions, which made me cau- 

 tious to insert nothing that might inti- 

 mate the contrary. For although suc- 

 cessive refractions which are made all 

 in the same way do necessarily more and 

 more augment the errors of the first re- 

 fraction; -yet it seemed not impossible 

 for contrary refractions so to correct 

 each other's inequalities, as to make their 

 difference regular, and if that could be 

 conveniently effected, there would be no 

 further difficulty. Now to this end I 

 examined what may be done not only by 

 glasses alone, but by a complication of 

 divers successive mediums ; as by two 

 or more glasses or crystals, with water 

 or some other fluid between them ; all 

 which may together perform the office of 

 one glass, especially of the object-glass, 

 on whose construction the perfection of 

 the instrument chiefly depends. But 

 what the results in theory or by trials 

 have been, I may possibly find a more 

 proper occasion to declare." 



Jn this passage he hints at the prin- 

 ciple on which achromatic telescopes 

 depend, and even the manner of ap- 

 plying that principle in their construc- 

 tion, and yet fifty years of his life 

 after this employed in perfecting his 

 theory, seem only to have confirmed his 

 error. 



(38.) Abandoning all further inquiry 

 into the methods of improving refracting 

 telescopes, Newton, at the part of his 

 Optics to which we have now arrived, 

 proceeds to explain his contrivances for 

 the construction of a reflecting telescope. 

 In the end of a tube he placed a con- 

 cave spherical reflector, which he con- 

 structed of metal, and polished with his 

 own hands. The image from this was 

 deflected by another plane reflector 

 placed in the axis of the tube, so as to be 

 received by an eye-glass in the side of 

 the tube at which it was viewed by the 

 observer. He suggests the possibility of 



