. THE MICROSCOPE. 49 



in the form of a circular piece of glass, that is dropped into 

 the eye-piece when required, or the glass slip may, as re- 

 commended by Mr. Jackson, be mounted in a brass plate 

 capable of being pushed into the eye-piece through a slit 

 in its side. The former is preferable, but it will not neces- 

 sarily follow that, if the micrometer be dropped into the 

 eye-piece, its lines will be clearly seen. They require to 

 be exactly in the focus of the upper lens of the eye-piece, 

 and to bring this about, an optician may have to fit the 

 micrometer into a tube at such a height, that when it is 

 dropped into the ocular, and rests on the stop, the lines 

 will appear sharp and distinct. The micrometer lines 

 usually form a series like that represented in Fig. 40, and 

 for the purpose of ordinary measurement this is doubtless 

 all that is necessary. It is, however, 

 of great advantage to have the micro- 

 meter plate divided into squares, each 

 square being the breadth of ten of the 

 finest divisions of the scale (Fig. 43). FlG ; 40.- Ordinary form 



. . ,.' of eye-piece micrometer, 



A micrometer so constructed* is indis- with a human blood cor- 

 pensable for counting the blood-cor- 

 puscles (Fig. 43), and also serves as the 

 square of glass for drawing, alluded to in 58 d. In 

 measuring the breadth of the human coloured blood cor- 

 puscles, use the J-in. objective, elongate the tube, 

 and substitute the micrometer in the ocular. Bring a 

 single coloured corpuscle into focus. It is seen through 

 the micrometer lines (Fig. 40). Suppose its broad diameter 

 covers four of the smallest divisions : push in the tube ; the 

 corpuscle does not now cover so many spaces ; hence the value 

 of the divisions of the ocular micrometer varies with the mag- 

 nifying power, and it is necessary to determine their value 

 for all the powers employed. Therefore substitute a stage 

 micrometer for the blood. If an English one be used, 

 focalise a 10 1 00 inch division. Elongate or push in the 

 tube until it covers just ten divisions of the ocular micro- 

 meter, then evidently each division must, with this magni- 



* Such micrometers are made by Mr. Bryson, 60 Princes Street, 

 Edinburgh. 



E 



