spherical and Chromatic Aberration. By Dr. Pioyston-Pigott. 131 



no way of finding this mathematically, except by the calculation 

 of the aberration of the red and violet rays from the spherical 

 formula involving their indices of refraction and radii of surfaces, 

 and thickness. 



The chromatic aberration is finely shotvn by a very large 

 burning-glass. The author possesses one of 8 inches in diameter. 

 If the whole be covered up except a half inch rim, the image of the 

 sun will be seen of different colours on a semi-transj^arent screen 

 held at the various foci. Each colour produces its own brilliant 

 focal image in order, and their exact positions measure in some 

 degree the dispersion of the glass. If now the whole be covered 

 up except an inch in the centre, the order will be the same as 

 before, but their former positions are altered. The difference 

 between the positions of the red image of the sun, for instance, is 

 the spherical aberration of the red for the given glass and curvature : 

 and the variation of the position of the violet image of the sun for 

 the marginal and central rays of the burning-glass is the spherical 

 aberration of the violet ; and is absolutely identical with the 

 spherical aberration of the marginal rays of that kind of light 

 which has the same refractive power as the red or violet in 

 question. 



The chromatic dispersion, or, much better, the dispersion, is best 

 shown by the spectroscope, formed of several accurately constructed 

 symmetrical prisms, and can only be very correctly measured by 

 using plane instead of spherical surfaces (very perfectly formed 

 to bend the rays). Barlow succeeded in determining the chro- 

 matic dispersive power roughly by measuring the distance to 

 hundredths of an inch, by which lenses of different materials 

 formed rude achromatic images when separated by a measurable 

 interval, the image of a black cross on white paper being used. 

 This method I take the liberty to call rough, as it cannot be com- 

 pared for a moment to the delicate method of measuring wave- 

 lengths as employed in the best spectroscopes. 



The very curious laws of dispersion revealed by this modern in- 

 strument, depending both on the intrinsic quality of the light and 

 the media through which it is transmitted, can be investigated now 

 under circumstances of unprecedented precision and advantage. The 

 detection of the velocity of motion, for instance, of Sirius as re- 

 ceding from or approaching the sun, is an example of the most 

 subtle process of analysis yet exhibited to mankind. Dispersion 

 (and its correlations) is now one of the most interesting depart- 

 ments of modern physics. 



There can be no doubt that every case of marginal aberration 

 in a coloured ray, though identical in the laws of its refraction with 

 what is called spherical aberration, has yet further qualities depen- 

 dent on its source. Thus the aberrations of the blue rays pro- 



