White Cloud Illumination, 293 



transparent objects, and which ought to be as uniform as 

 possible in brilliance throughout its whole extent. A single 

 piece of ground glass gave upon trial an imperfect result, and 

 the writer then went to Mr. Browning's factory, and selected 

 a double concave lens, such as is used in telescope manufacture, 

 about 1^ inches in diameter. On one side this lens was 

 ground with emery, and the other polished as usual. A flat 

 disc of plate glass of the same size was also ground on one 

 surface, and placed upon the lens so that the two ground 

 surfaces were in contact. Mr. Browning then, by the writer's 

 direction, cemented the two glasses by their edges, and 

 mounted them in a brass cell fitting the substage apparatus 

 of the microscope. 



Nothing could be better than the effect thus obtained. 

 Objects, such as insects mounted whole, the head and antennae 

 of the plumed gnat, delicate, transparent, anatomical prepara- 

 tions, came out beautifully with the binocular and powers of 

 11 to 3 inches. The extreme softness of the light was com- 

 mended by several experienced observers, and persons with 

 sensitive eyes declared that they could view the objects steadily 

 without fatigue. 



It was obvious that by grinding one surface of the lens, 

 its action as a lens must be greatly interfered with, but the 

 experiment was made in the belief that it would still exert a 

 more dispersive action than a flat piece of glass. To test this, 

 Mr. Browning was asked to grind two flat discs of plate glass 

 on one side only, and they were found to perform so nearly as 

 well as the combination of flat disc and lens, that the writer is 

 disposed to recommend them instead, as they are cheaper. 

 Comparative experiments seemed to show a slight superiority 

 in the lens combination, which is certainly less pervious to 

 light. When one disc of ground glass only was tried, the 

 effect was far less satisfactory than with the two. 



These discs should be cemented together while their ground 

 surfaces are fresh. All dust is thus excluded, and the per- 

 manent whiteness of the field ensured. Different kinds of 

 glass grind with different degrees of whiteness, and that which 

 gives the most snowy-looking surface should be selected. 



Almost every microscopist has doubtless used ground glass 

 for this purpose ; but those who try two ground surfaces 

 instead of one, and adopt the method here pointed out for 

 keeping them clean, cannot fail to be pleased with the result. 



