208 USE OF THE MICROSCOPE. 



Before leaving the subject of the illumination of micro- 

 scopic objects, it may be as well here to lay down certain 

 rules, which, for the accurate display of minute structures, 

 should be fully carried out. Sir David Brewster,* the greatest 

 living authority in these matters, has insisted on the follow- 

 ing; some of these, with a few alterations suitable to our 

 modern microscopes, we will adopt : — 



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

 except that 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 to be in a viscid state, which is frequently 

 the case. 



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

 lubricating 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 looking downwards, as 

 into the compound microscope arranged vertically, the fluid 

 has a tendency to flow towards the pupil, and injure the dis- 

 tinctness of the vision. 



4. If the microscopic object is longitudinal like a fine hair, 

 or consists of longitudinal stripes, the direction of the lines or 

 stripes should be towards the observer's body, in order that 

 their form may be the least injured by the descent of the 

 lubricating fluid over the cornea. 



5. The field of view should be contracted, so as to exclude 

 every part of the object, excepting that which is under imme- 

 diate examination. 



6. The light which is employed for the purpose of illumin- 

 ating the object should have as small a diameter as possible. 

 In the day time it should be a single hole in the window 

 shutter of a darkened room, and at night it should be an 

 aperture placed before an argand lamp. To effect this last 

 desirable object, either the diaphragm represented by fig. 63, 

 or the chimney shade described at page 153, may be 

 employed. 



* Treatise on the Microscope, p. 138. 



