464 POPULAR SCIENCE MONTHLY. 



was detected to be no other, than that light consists of rays differently 

 refrangible, which, without any respect to a difference in their inci- 

 dence, were according to their degrees of refrangibility, transmitted 

 towards divers parts of the wall. 



When I understood this, I left off my aforesaid glass works, 

 for I saw, that the perfection of telescopes was hitherto limited, not 

 so much for want of glasses truly figured according to the prescriptions 

 of optic authors, (which all men have hitherto imagined,) as because 

 that light itself is a heterogeneous mixture of differently refrangible 

 rays. So that, were a glass so exactly figured, as to collect any one 

 sort of rays into one point, it could not collect those also into the 

 same point, which having the same incidence upon the same medium 

 are apt to suffer a different refraction. Nay, I wondered, that seeing 

 the difference of refrangibility was so great, as I found it, telescopes 

 should arrive to that perfection they are now at. For measuring 

 the refractions in one of my prisms, I found that supposing the 

 common sine of incidence upon one of its planes was 44 parts, the 

 sine of refraction of the utmost rays on the red end of the colours, 

 made out of the glass into the air, would be 68 parts, and the sine 

 of refraction of the utmost rays on the other end 69 parts: so that 

 the difference is about a 24th or 25th part of the whole refraction; 

 and consequently, the object glass of any telescope cannot collect all 

 the rays which come from one point of an object, so as to make them 

 convene at its focus in less room than in a circular space, whose 

 diameter is the 50th part of the diameter of its aperture; which is 

 an irregularity, some hundreds of times greater than a circularly 

 figured lens, of so small a section as the object glasses of long telescopes 

 are, would cause by the unfitness of its figure, were light uniform. 



This made me take reflections into consideration, and finding them 

 regular, so that the angle of reflection of all sorts of rays was equal 

 to their angle of incidence; I understood that by their mediation 

 optic instruments might be brought to any degree of perfection 

 imaginable, provided a reflecting substance could be found, which 

 would polish as finely as glass, and reflect as much light as glass 

 transmits, and the art of communicating to it a parabolic figure be 

 also attained. But there seemed very great difficulties, and I have 

 almost thought them insuperable, when I further considered, that 

 every irregularity in a reflecting superficies makes the rays stay five 

 or six times more out of their due course, than the like irregularities 

 in a refracting one: so that a much greater curiosity would be here 

 requisite, than in figuring glasses for refraction. 



Amidst these thoughts I was forced from Cambridge by the in- 

 tervening plague, and it was more then two years before I proceeded 

 further. But then having thought on a tender way of polishing, 



