2 Marvels of Pond-Life. 



invidious. We shall not, therefore, indulge in the praises 

 of particular firms, but simply recommend any reader 

 entering upon microscopic study to procure an achro- 

 matic instrument, if it can be afforded, and having at 

 least two powers, one with a focus of an inch or two 

 thirds of an inch, and the other of half or a quarter. 

 Cheap microscopes have usually only one eye-piece, 

 those of a better class have two, and the best are fur- 

 nished with three, or even more. 



The magnifying power of a compound microscope 

 depends upon the focal length of the object-glass (or 

 glass nearest the object), upon the length of the tube, 

 and the power of the eye-piece. With regard to object- 

 glasses, those of shortest focal length have the highest 

 powers, and the longest eye-pieces have the lowest 

 powers. The body of a microscope, or principal tube 

 of which it is composed, is, in the best instruments, 

 about nine inches long, and a draw tube, capable of 

 being extended six inches more, is frequently useful. 

 From simple optical principles, the longer the tube the 

 higher the power obtained with the same object-glass; 

 but only object-glasses of very perfect construction will 

 bear the enlargement of their own imperfections, which 

 results from the use of long tubes; and consequently for 

 cheap instruments the opticians often limit the length of 

 the tube, to suit the capacity of the object-glasses they 

 can afford to give for the money. Such microscopes 

 may be good enough for the generality of purposes, but 

 they do not, with glasses of given focal length, afford 

 the same magnifying power as is done by instruments 

 of better construction. The best and most expensive 



