ILLUMINATION. 269 



the sun. Eeflected light from walls, &c., may also be of value 

 under certain circumstances, yet its most favourable employment 

 is limited to certain periods of the day. Hence, in towns, the 

 upper stories of houses which allow of our utilizing the light near 

 the horizon are preferable, and ground-floor dwellings in narrow 

 streets should be avoided. 



The most favourable day -light illumination is, in our opinion, 

 uniform grey cloud ; the pure blue sky also gives an agreeable 

 light, but is not intense enough to develope the fullest power of 

 the highest objectives. In difficult investigations, therefore, a 

 bright white or yellowish white wall illuminated by the sun 

 is more serviceable. Quickly-passing clouds are unfavourable, 

 because the eye is soon fatigued by the continual change of 

 brightness and colour. Direct sun-light should be avoided, or 

 subdued by blinds. 



As regards placing the Microscope, it is generally stated that a 

 more favourable effect of light is obtained at a distance of six to 

 nine feet from the window for critical work than at the usual 

 distance of about three feet. 



According to what we have already stated on illumination, 

 the distance from the window is of importance only when 

 the aperture of the incident cone of light is limited thereby, 

 which is not the case on the application of small diaphragms such 

 as are ordinarily used for high amplifications. In using oblique 

 light, we must under all circumstances be careful that the light 

 reflected from the table and stand is excluded, which may be 

 effected by a vertical diaphragm specially contrived, or by lateral 

 displacement of the usual diaphragm. In many experiments the 

 light incident from above upon the object should be excluded by 

 holding the hand or a screen before it. In short, the illumination 

 should in all difficult cases be so arranged that no other light 

 reaches the field of view than the incident cone of definite aperture 

 and inclination. 



What aperture or inclination of the incident cone of light may be 

 most advantageous, must of course be found by trial in every 

 case. The mirror, or if necessary the whole instrument, is moved 

 in various directions, the observer at the same time looking through 

 the eye-piece, and the effect of the different diaphragms is noticed ; 

 or they are moved up and down in the direction of the axis. These 

 experiments are continued until the most favourable kind of illu- 



