34 



VIEWING AND ILLUMINATING 



Fig. 19. 



Fig. 20. 



22. The following rules, as given by Brewster, may be laid down 

 respecting the illumination of microscopic objects, and the method of 

 viewing them : 



1. The eye should be protected from all extraneous light, and 

 should not receive any of the light which proceeds from the illumina- 

 ting centre, excepting that portion of it which is transmitted through 

 or reflected from the object. 



2. Delicate microscopical observations should not be made when 

 the fluid, which lubricates the cornea of the observer's eye, happens 

 lo be in a viscid state, which is frequently the case. 



3. The figure of the cornea will be least injured by the lubrica- 

 ting fluid, either by collecting over any part of the cornea, or moving 

 over it when the observer is lying on his back, or standing vertically. 

 When he is looking downwards, as into the compound vertical micro- 



