Otf Tttfi MICROSCOPE. 41 



instrument is at once converted into a monocular, equal in 

 every respect to any that are made specially in that way. The 

 fine motion is by a lever, which moves the nose-piece, to which 

 the objective is attached. The instrument is furnished with a 

 well-made mechanical stage, and we would call the special 

 attention of the reader to a point, which we have already noted, 

 viz., the abundant room which is found below the stage for the 

 application of accessory illuminating apparatus. The sub-stage, 

 which has very delicate means of adjustment for focussing and 

 centering, has considerable range, and affords facilities which 

 cannot possibly be combined with what may be called the 

 "dumpy " models. 



Binocular Eye-Piece A very valuable and efficient 

 means of converting an ordinary monocular microscope into a 

 binocular, has been devised by Mr. Tolles, of Boston. But ite 

 price, $80, places it beyond the reach of "beginners," for 

 whom this work is specially intended. 



OBJECTIVES. 



The modern compound microscope owes almost all its value 

 to the high degree of perfection which has been attained in the 

 construction of the objectives used with it Some of the old 

 microscope stands were quite as elaborate, and quite as costly, as 

 anything that can be found in the workshops of our modern 

 opticians, but from the fact that the objectives were little else 

 than simple lenses, their value as instruments of research was 

 of a very low degree. This being the case, it is of some impor- 

 tance to the microscopist that he should have at least a fair 

 understanding of the causes to which the superiority of 

 modern objectives is due. 



When we use a simple glass lens as an objective for a com- 

 pound microscope, we find on attempting to examine objects 

 under powers of more than one hundred diameters, the following 

 defects and difficulties: The field of view is so dimly illuminated 

 that objects are seen with difficulty; the outlines of the different 

 parts, instead of being sharp and clear, are thick and hazy; 

 several of the lines are fringed with brilliant colors, but colors 



