206 Obituary Notices of Fellows deceased. 



It may be remarked that on any mechanical theory the reflection 

 from an infinitely thin plate must tend to vanish, and therefore that 

 a contrary conclusion can only mean that the theory has been applied 

 incorrectly. 



A not uncommon defect of the eye, known as astigmatism, was first 

 noticed by Airy. It is due to the eye refracting the light with 

 different power in different planes, so that the eye, regarded as an 

 optical instrument, is not symmetrical about its axis. As a consequence, 

 lines drawn upon a plane perpendicular to the line of vision are 

 differently focussed according to their direction in that plane. It may 

 happen, for example, that vertical lines are well seen under conditions 

 where horizontal lines are wholly confused, and vice versa. Airy had 

 shown that the defect could be cured by cylindrical lenses, such as are 

 now common ; but no convenient method of testing had been proposed. 

 For this purpose Stokes introduced a pair of piano-cylindrical lenses 

 of equal cylindrical curvatures, one convex and the other concave, and 

 so mounted as to admit of relative rotation. However the components 

 may be situated, the combination is upon the whole neither convex nor 

 concave. If the cylindrical axes are parallel, the one lens is entirely 

 compensated by the other, but as the axes diverge the combination 

 forms an astigmatic lens of gradually increasing power, reaching a 

 maximum w^en the axes are perpendicular. With the aid of this 

 instrument, an eye, already focussed as well as possible by means (if 

 necessary) of a suitable spherical lens, convex or concave, may be 

 corrected for any degree or direction of astigmatism; and from the 

 positions of the axes of the cylindrical lenses may be calculated, by a 

 simple rule, the curvatures of a single lens which will produce the 

 same result. It is now known that there are comparatively few eyes 

 whose vision may not be more or less improved by an astigmatic 

 lens. 



Passing over other investigations of considerable importance in 

 themselves, especially that on the composition and resolution of 

 streams of polarised light from different sources, we come to the great 

 memoir on what is now called Fluorescence, the most far-reaching of 

 Stokes' experimental discoveries. He "was led into the researches 

 detailed in this paper by considering a very singular phenomenon 

 which Sir J. Herschel had discovered in the case of a weak solution of 

 sulphate of quinine and various other salts of the same alkaloid. 

 This fluid appears colourless and transparent, like water, when viewed 

 by transmitted light, but exhibits in certain aspects a peculiar blue 

 colour. Sir J. Herschel found that when the fluid was illuminated by a 

 beam of ordinary daylight, the blue light was produced only throughout 

 a very thin stratum of fluid adjacent to the surface by which the light 

 entered. It was unpolarised. It passed freely through many inches of 



