160 THE MICROSCOPE. 



When you look through the instrument, be sure to 

 place your eye quite close to the eye-piece, otherwise the 

 whole field of view will not be visible ; and observe, more- 

 over, if you see a round disc of hght, at least when the 

 object is not on the slider-holder : if you do not, it is a 

 sign that something is wrong; perhaps the body is not 

 placed directly before the stage aperture, or may not be 

 properly directed towards the light. Use the smallest 

 amount of light possible, if you work for any length of 

 time. Choose a steady light, with a shade to protect the 

 eyes, one of the old-fashioned fan-shades will be found 

 useful for this purpose : use the eyes alternately. Sit in a 

 comfortable position, and bring the instrument to the 

 proper angle, which will prevent congestion of the eyes ; 

 this is indicated if the microscopist is annoyed with little 

 moving objects apparently floating before them : if the 

 eye-lashes are reflected from the eye-glass, you are looking 

 upon the eye-glass instead of through it. Take care that 

 the mirror is properly arranged. 



The following are Sir David Brewster's excellent direc- 

 tions for viewing objects : — 



" First. Protect the eye from all surrounding light, let- 

 ting only the rays which proceed from the illuminated 

 centre of the object fall upon it. 



" Secondly. Delicate observations should not be made 

 when the fluid which lubricates the cornea is in a viscid 

 state, or there is any irritation or inflammation about any 

 part of the eye. 



" Thirdly. The best position for microscopic observations 

 is with the microscope bent to such an angle with the 

 body, that the head may always remain in a natural and 

 easy attitude ; consequently, the worst position would be 

 that which compels us to look downwards vertically. 



" Fourthly. If we lie horizontally on the back, parallel 

 markings and lines on objects will be seen more perfectly 

 when their direction is vertical, or in a contrary direction 

 to that in which the lubricating fluid descends over the 

 cornea of the eye. 



" Fifthly. Only a portion of the object should be viewed 

 at one time, and every other part excluded. The light 

 which illuminates that part should be admitted through a 



