138 THE AMERICAN MONTHLY [May 



141 pages, illustrated by three double-colored plates, with 

 au elaborate article on the Structure and Metamorphosis 

 of the larva, Spongilla lacustris. 



Second-hand Objectives. — Do not on any account buy 

 second-hand apochromatics unless they have first been re- 

 turned to Messrs. Zeiss to examine, and, if necessary, to 

 rectify. Messrs. Zeiss only guarantee their lenses against 

 atmospheric deterioration. 



Work. — We propose here to deal with the elementary 

 management of the microscope and its accessories. The 

 modern microscope has become an instrument of such ex- 

 quisite precision that the beginner generally finds himself 

 at the very outset in need of explanation and assistance 

 in this respect. A clear understanding of what he is do- 

 ing is necessary if he is to obtain the best results. In our 

 medical and other laboratories too little attention is often 

 given to these matters, and it follows, therefore, that the 

 student is not really familiar with the use of his instru- 

 ment, and obtains more or less imperfect results. The gen- 

 eral use of the sub-stage condenser has revolutionized 

 microscopy, and we would here call attention most strong- 

 ly to the fact that the proper management of the illumin- 

 ation of the object is a matter requiring as much or even 

 o-reater care and attention than the actual focussing of the 

 objective itself. This last is comparatively a simple mat- 

 ter, save where corrections have to be made for diverse 

 thicknesses of cover-glasses. 



We will assume that the reader has provided himself 

 with a microscope well fitted up. He will place his mi- 

 croicope on the table before him, and incline it at a com- 

 fortable angle for looking down the tube. Of course there 

 is a certain amount of work that can only be done with 

 the microscope in a vertical position ; but when the object 

 will admit of it, it is a great convenience and rest to the 

 muscles of the neck if the microscope be slightly inclined. 

 This is a point that makers of even the less expensive 



