ILLUMINATION. xxxi 



of glass of such colour as to intercept or check the objectionable rays. As these rays are 

 of a yellow or reddish-yellow colour, the colour of the glass must be blue or purplish blue j 

 but the exact shade must be obtained by experiment. Thus: the lamp, or whatever 

 source of artificial light it may be, is lighted in the daytime, and the light transmitted 

 through the microscope by reflection in the ordinary way, when its intensely yellowish 

 colour is very obvious. Pieces of glass of different colours are then separately placed at 

 right angles to the path of the rays from the lamp to the mirror, either close to the flame 

 (in the form of an ordinary lamp-glass), upon the face of the mirror itself, beneath the 

 stage, or in an extra head of the side-condenser. If the glass be of the proper tint, and be 

 placed at the proper distance from the light, and in the proper situation, the field will 

 appear as white as the light of the clouds, which may be easily proved by altering the 

 inclination of the mirror so as to reflect the light of the clouds and the lamp alternately. 



It may be remarked that the nearer the coloured glass is placed to the flame the less 

 apparent effect is produced, i. e. the more will the yellow colour be perceptible, and 

 vice versa. If the field still appear yellow, the glass is not of sufficiently deep colour ; if 

 it appear blue, the colour of the glass is too deep. The first method, or that of mixing 

 some substance with the combustible (oil, tallow, &c.) capable of evolving a light of the 

 requisite tint to form white with the yellow of the artificial light would be far preferable 

 to the latter method ; but I am not aware that any experiments have been made to carry 

 out this idea. It would have two great advantages, viz. that there would be no diminu- 

 tion of light, and that the entire apartment would be illuminated by a light equivalent to 

 that of ordinary day. The second method has one objection, which is, that it intercepts 

 a large quantity of the light, so that in the examination of those objects with high powers 

 which require intense illumination, or where much of the light is arrested by stops, it is 

 decidedly objectionable. The advantages which the use of the blue glass possesses are, 

 that it softens the h'ght very much, and that it enables the observer to discriminate 

 between colours as in ordinary daylight. 



A few years after the publication of the above method, a patent was taken out for the 

 construction of lamp-glasses of a blue colour ; but they are of little service, merely slightly 

 softening the light, or intercepting a small proportion of the yellow rays. Perhaps, 

 some day, a small electric lamp, worked by clock-work, will be invented. 



The proper way would be to "flash " the suitably tinted blue glass upon one side of a 

 pale blue lamp-glass, so that, by simply turning the glass round, the light might be trans- 

 mitted through either of the differently coloured portions. Rainey's " Light-modifier " 

 acts upon this principle. Numerous other pieces of apparatus and ingenious contrivances 

 will be found described and mostly figured in the last edition of Carpenter's ' Microscope/ 

 or in Beale's ' How &c.' 



The illumination is of importance to the microscopic observer in another sense, i. e. in 

 regard to the injury of sight. The great point here is to avoid too powerful a h'ght. An 

 eminent French philosopher became blind in experimenting upon the duration of powerful 

 impressions upon the retina. In some instances, sun-light has been used in microscopic 

 investigation ; the greatest care must then be taken to use screens or diaphragms to temper 

 the lisrht. 



