12 I. THE COMPOUND MICROSCOPE STRUCTURE OF STARCH. 



serious injury to object or objective than with the hand movement. 

 Once more, always focus up, i.e., wind the tube down while 

 watching the position of the objective from the side, and when 

 low enough, place the eye to the eye-piece, and focus slowly 

 upwards with the screw while carefully looking for the object, 

 and perfect the focussing by means of the micrometer screw fine 

 adjustment. Whatever microscope you use make it a standing 

 rule to focus upwards, not downwards. When you have become 

 an expert observer you may do as you please. 



After we have determined, with the slight magnification of our 

 weaker objective, the existence of small grains in the field of view 

 of the microscope, and have noted, for future use, the distance of 

 this low power objective from the object, i.e., its focal or w r orking 

 distance, we leave the object-slide unmoved upon the stage, but 

 withdraw the tube from its guiding sheath, unscrew the weak ob- 

 jective and screw in a stronger one, or twist round the nose-piece, 

 but in no case using a stronger power than about J inch of the 

 English makers. We then replace the tube in its sheath, and 

 push it down so far that the objective now very nearly touches the 

 cover-class. We again endeavour to catch sight of the object 

 while raising the tube in its sheath. With the higher magnifying 

 power it must, however, be withdrawn much more slowly than 

 before. As the preparation has lain unmoved upon the stage we 

 know that the object should be found in the field of view of the 

 microscope. When the grains have become visible with this 

 coarse adjustment, we perfect the fine focussing with the micro- 

 meter screw as before. We shall find the working or focal 

 distance of the stronger objective is considerably less than that 

 of the weaker one, and always less (for glasses of the same make) 

 in proportion as the objective is stronger. Again take careful 

 note of the working distance. As the magnification is now 

 stronger the illumination of the figure appears feebler ; it may be 

 possible to increase the light by adjustment of the mirror. We 

 shall, however, find it very easy to turn the light altogether from 

 the object ; and a very convenient way of getting the light 

 properly directed before commencing to work is by removing the 

 objective altogether, and looking through the eye-piece and tube 

 at the mirror. If this is properly arranged, the whole field of view 

 of the eye-piece will appear as uniformly bright as possible. 



With some practice it is possible, in the absence of a double 

 nose-piece, to change the objectives without completely with- 



