PARKED MICROSCOPE LAMP. 



95 



lighted, this vessel is filled with water, and so prevents the 

 radiation of heat upon the observer's head. The felt requires 

 moistening about once every five hours. 



The light of the sun, a white cloud, or the electric light, 

 which the author has used, and will illustrate in a future 

 chapter, each give a light of remarkable purity. This is 

 not the case, however, with the light from gas or from oil 

 lamps. These last, especially gas 

 sources of illumination, give a very 

 objectionable yellow tone, while 

 some tints are nearly suppressed. 

 This effect has been noticed by all 

 observers, and in 1872 Mr. Collins 

 produced a light-corrector, and 

 exhibited the same at a soire'e at 

 the Quekett Club. It consists of a 

 brass stage-plate with a groove in 

 which rotates a diaphragm of 4 

 apertures one open, one fitted with 

 a finely ground glass, while the 

 others are fitted with two different 

 tints of blue. Rainey produced a light-modifier before 

 this, but it was of such construction that it required 

 fitting to each microscope; that of Mr. Collins, on the 

 other hand, can be used with any instrument, and without 

 fitting. 



The effect of the blue glass is to effectually correct the 

 yellowness proceeding, from all artificial illumination, 

 rendering the light soft and agreeable, as well as to im- 

 prove the definition. To produce this effect, the writer 

 uses a simple 3 in. x i in. slide of blue glass, such as is 

 used by the chemist for the qualitative analysis of potash 

 salts. It was obtained from Messrs. J. J. Griffin and Sons, 

 Garrick Street, Covent Garden. 



FIG. 85. 



