316 W. K. BRIDGMAN ON THE 



margin to that which has been stopped out in the centre, rendering 

 objects beautifully stereoscopic and clear. 



The next consideration is as to the quality of the light. The 

 dazzling and detail-obliterating glare of the sun's noonday rays is 

 ■well known to be exceedingly trying as well as injurious to the eye- 

 sight, and the concentrated rays from a lamp or even of daylight is 

 only a question of degree as to its inapplicability to distinct vision, 

 as well as to the safety of the eyesight. It has frequently been 

 remarked that visitors, on first looking into an instrument, have 

 exclaimed, " Oh, what a brilliant light ! it dazzles the eye so that I 

 can see nothing else yet" and which is unfortunately but too 

 commonly true to the letter. Instead of the light being made sub- 

 ordinate to the object, it is too often the chief thing which arrests 

 the attention. An artist, painting a picture, takes care to subdue 

 his background and make it the means of leading the eye towards 

 his principal object, and the same ought to be done in the micro- 

 scope. The brightness and the tint of the field should be unobtru- 

 sive, and serve to throw the object into distinctness, and cause the 

 eye to be immediately attracted to it, leaving the light itself to make 

 its presence known by feeling rather than by appealing to the eye. 



With the reflecting illuminator the silvery whiteness was subdued 

 by the yellow of a small portion of gold as an alloy ; but with 

 refraction, the only course open is by means of a tinted transparent 

 medium, and for this purpose blue glass has been commonly 

 employed, although it is highly objectionable in more ways than one. 

 Having tried all the tints of the spectrum, and a variety of combi- 

 nations in addition, it has been found, practically, that there is no 

 other tint to at all compare with the neutral tint occurring between 

 the green and the yellow, with a very slight predominance of the 

 former. This gives a beautifully cool and agreeable tone to the 

 background of the field ; at the same time that, by absorbing the 

 excess of light, it reduces bright sunshine to clear daylight, and 

 materially improves the distinctness of definition. 



My own experience leads to the conclusion that by very far the 

 best place for the tinted glass is between the lens and the object ; 

 but if it be made to rest upon the face of the condenser, so as to 

 have its surfaces parallel with the plane surface of the latter, it will, 

 by its refraction, materially interfere with the definition : its proper 

 position being, as shown in the diagram (Fig. 3), at right angles to 

 the course of the ray after it has left the condenser. In this direc- 



