Ch. V] MEASURING WITH THE MICROSCOPE 157 



the center of the condenser (§ 229), if the object (lantern slide of the 

 lines) is always placed exactly the same distance in front of the mi- 

 croscope the real image formed by the condenser will be of the same 

 size, and hence have the same valuation for micrometry regardless of 

 the power of the objective or the length of tube used. It is a very 

 convenient method of micrometry for all coarser objects, but not exact 

 enough for the finer objects. A movable scale or filar ocular microm- 

 eter should be used for the most exact work. 



Example of an actual measurement by means of the condenser 

 image: The long axis of a red corpuscle of Necturus measured 0.61 

 of a space of the condenser image. As each space represents 0.075 

 mm. the length of the corpuscle is: 0.061 X 0.0.075 = °-°4575 mn "i- 

 or4S-75/ x (see Chamot, pp. 155-157). 



§ 256. Remarks on micrometry. — In using adjustable objectives 

 (§ 31, 134) the magnification of the objective varies with the position 

 of the adjusting collar, being greater when the adjustment is closed, 

 as for thick cover-glasses, than when open, as for thin ones. This 

 variation in the magnification of the objective produces a corresponding 

 change in the magnification of the entire microscope and the ocular 

 micrometer valuation; therefore it is necessary to determine the 

 magnification and ocular micrometer valuation for each position of 

 the adjusting collar. 



While the principles of micrometry are simple, it is very difficult 

 to get the exact size of microscopic objects. This is due to the lack 

 of perfection and uniformity of micrometers and the difficulty. of 

 determining the exact limits of the object to be measured. Hence, 

 all microscopic measurements are only approximately correct, the 

 error lessening with the increasing perfection of the apparatus and 

 the skill of the observer. 



A difficulty when one is using high powers is the width of the lines 

 of the micrometer. If the micrometer is perfectly accurate half the 

 width of each line belongs to the contiguous spaces, hence one should 

 measure the image of the space from the centers of the lines bordering 

 the space, or, as this is somewhat difficult in using the ocular mi- 

 crometer, one may measure from the inside of one bordering line and 

 from the outside of the other, that is, from the right side of all the 



