CH. II] DAYLIGHT GLASS 49 



All forms of artificial light have been used at some time for micro- 

 scopic work. For photography and for drawing (Chs. VI-VII) the 

 arc light has been found most satisfactory. For the usual observa- 

 tional work with the microscope the effort has been made for a long 

 time to get an artificial light which should approximate daylight as 

 closely as possible. This desire for artificial daylight is natural, as the 

 eye has been created or developed for daylight, and any form of light 

 differing in a marked degree from daylight does not give standard 

 color values, and is liable to cause eye fatigue if some parts of the 

 visible spectrum are markedly brighter than with daylight. In all 

 of the ordinary forms of artificial light, the relative intensity toward 

 the red end of the spectrum is very much greater than with daylight 

 (fig. 36) ; hence color values are distorted, and with most people the 

 excessive red intensity produces a glare and lack of contrast which is 

 trying to the eyes. 



92. Artificial daylight. For the production of artificial day- 

 light it is obvious from the curve here shown that there are two pos- 

 sible means: (i) The selection of two kinds of artificial light in which 

 the lack in one is made good by the excess in another, and by mixing 

 these in the right proportions the resulting light will have the same 

 relative intensity in different parts of the spectrum as is found in sun- 

 light. This is the "addative" method and has been quite success- 

 fully realized by combining a mercury arc light with its deficiency in 

 the red, but its richness in intensity in the blue end of the spectrum, 

 with a mazda incandescent lamp with its excessive red intensity. 

 If these two lights are enclosed in a glass globe, and the right amount 

 of each used, very good daylight is produced. 



(2) As there is excessive intensity in the red part of the spectrum 

 it is evident that if this excess can be absorbed by a light filter of some 

 kind, then also the relative intensity of the light will be like that of 

 natural daylight. This is the " sub tractive " method, and is the 

 method employed wherever a light filter or colored liquid, colored 

 gelatin, colored glass, or a combination is used. From time im- 

 memorial various colored liquids like solutions of copper salts and 

 colored glasses have been used to whiten the artificial light. 



During the last few years, however, the problem has been solved, 



