CHAPTER III. 



PRELIMINARY DIRECTIONS ILLUMINATION ACCESSORY APPARATUS 



ACHROMATIC ILLUMINATOR GILLETT'S CONDENSER PREPARING AND 



MOUNTING OBJECTS POLARISED LIGHT BINOCULAR INSTRUMENT 



PHOTOGRAPHIC DRAWING, ETC. 



CAVING selected an apartment with a northern 

 aspect, and, if possible, with only one win- 

 dow, and that not overshadowed by trees 

 or buildings : in such a room, on a firm, 

 steady table, keep your instruments and ap- 

 paratus open, and at all times ready for ob- 

 servation. A large bell-glass will be found 

 of great service in keeping dust from a mi- 

 croscope when set up for use. In winter it 

 will be proper to slightly warm the instru- 

 ment before it is used, otherwise the per- 

 spiration from the eye will be perpetually 

 condensing on the eye-glass, thus greatly 

 impeding vision. Always begin the ex- 

 amination of your object with the lowest 

 power you have, unless it be very minute. 



As a general rule, large objects require low powers, and small ones 

 high powers : low powers show the whole or general view of an object, 

 the high ones only its parts in succession 5 and as the power in- 

 creases, so does the difficulty of finding the object and adjusting the 

 focus. When you clean the eye-glasses, do not remove more than 

 one at a time, and replace it before you touch another; by so doing 

 you will preserve the component glasses in their proper places : recol- 

 lect that if intermingled they are useless. Keep a piece of well- 

 dusted and very dry chamois leather, slightly impregnated with the 

 finest tripoli or rotten-stone powder in a small box, to wipe your glasses : 

 a small piece of dried elder-pith is preferred by some for the purpose. 



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, moreover, if you see a round disc of light, at least 



