ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



549 



Fig. 92 



" The Microscopes are provided with adjustable eye-pieces, to 

 render the division of the micrometer distinct for every eyesight, and 

 at the lower end is a short draw-tube, by means of which a small 

 alteration in the magnification can be effected, whereby the intervals 

 of the micrometer (previously determined by calculation) can be 

 brought to accurately harmonize with the division of the circle. 



" As the field of view, though as extensive as possible, cannot be 

 so large as to always include figures of the divided circle, an index 

 with a magnifying lens must be fixed at any desired point, by means 

 of which the reading of the angle up to the nearest division of the 

 circle is obtained, while the determination of the excess is effected by 

 the Microscopes." 



The divisions of the circle with which the Microscopes are used 

 are not carried to any great degree of minuteness. The degrees are, 

 for instance, divided into sixths, or 10', and the micrometer consists of 

 ten equal divisions, representing, therefore, minutes, and the latter 

 can then be mentally subdivided with great facility. An important 

 advantage is, the author considers, obtained by the small number of 

 graduations of the micro- 

 meter, which permits an 

 easy, rapid, and accurate 

 reading, which does not 

 occupy so much time as in 

 the case of verniers. 



Swift's Tank Micro- 

 scope. — This, Fig. 92, con- 

 sists of the stand of a bull's- 

 eye condenser to which are 

 attached two additional 

 short arms, the upper one 

 carrying the microscope- 

 tube and the lower a re- 

 volving cork-holder and 

 forceps for flowers or other 

 objects suitable for low 

 powers. The tube has a 

 rack-and-pinion movement 

 and the arm to which it is 

 attached can be raised or 

 lowered on the standard 

 and clamped in any posi- 

 tion. The tube can also be 

 rotated on the arm so as to be either vertical or horizontal, or it 

 can be removed from the arm altogether. 



Teasdale's Field Naturalist's Microscope. — This (Figs. 93 

 and 94) is made by Messrs. Field of Birmingham, and was 

 designed by Mr. W. Teasdale with the view of providing the 

 working microscopist with a really cheap and efficient dissecting 

 Microscope, and it may be readily certified that it fully accomplishes 

 these objects. "It is so simply and substantially made that it 



