350 Professor Tyndall [March 19, 



to the received laws of refractions, I expected should have been 

 circular. They were terminated at the sides with straight lines, but 

 at the ends the decay of light was so gradual, that it was difficult to 

 determine justly what was their figure, yet they seemed semi-circular. 



" Comparing the length of this coloured spectrum with its breadth, 

 I found it about five times greater ; a disproportion so extravagant, 

 that it excited me to a more than ordinary curiosity of examining 

 from whence it might proceed." This curiosity Newton gratified by 

 instituting a series of experimental questions, the answers to which 

 left no doubt upon his mind that the elongation of his spectrum was 

 due to the fact " that light is not similar or homogeneal, but consists 

 of difform rays, some of wJiicJi are more refrangible than others ; so that 

 without any difference in their incidence on the same medium, some 

 shall be more refracted than others ; and therefore that, according to 

 thQiT particular degrees of refi'angibility J they were transmitted through 

 the prism to divers parts of the opposite wall. When," continues 

 Newton, " 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 optick authors, as because that light itself is an 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." 



Goethe harped on this string without cessation. " The New- 

 tonian doctrine," he says, " was really dead the moment achromatism 

 was discovered. Gifted men, our own Kliigel for example, felt this, 

 but expressed themselves in an undecided way. On the other hand, 

 the school which had been long accustomed to support, patch up, and 

 glue their intellects to the views of Newton, had surgeons at hand 

 to embalm the corpse, so that even after death, in the manner of 

 the Egyptians, it should preside at the banquets of the natural 

 philosophers." 



In dealing with the chromatic aberration of lenses, Goethe proves 

 himself to be less heedful than usual as an experimenter. With 

 the clearest perception of principles, Newton had taken two pieces of 

 cardboard, the one coloured a deep red, the other a deep blue. 

 Around those cards he had wound fine black silk, so that the silk 

 formed a series of separate fine dark lines upon the two coloured 

 surfaces. He might have drawn black lines over the red and blue, 

 but the silk lines were finer than any that he could draw. Illuminat- 

 ing both surfaces, he placed a lens so as to cast an image of the 

 surfaces upon a white screen. The result was, that when the dark 

 lines were sharply defined upon the red, they were imdefined upon 

 the blue ; and that when, by moving the screen, they were rendered 

 distinct upon the blue, they were indistinct upon the red. A distance 

 of an inch and a half separated the focus of red rays from the focus 



