THE MICROSCOPE. oU 



procure an achromatic object-glass with shallow magnifiers, not- 

 withstanding the decided advantage to be derived from their use. 



The microscopes of M. Nachet, and M. Brunner, of Paris, 

 have been highly recommended. Those of the former which I 

 have seen, are about on a par with the instruments made by 

 Oberhauser, with the advantage of a larger stage. 



Mr. C. A. Spencer, of Canastota, New York, has succeeded 

 in manufacturing object-glasses, which are said to have an 

 angle of aperture even greater than the best English achro- 

 matics. With them he succeeded in resolving the fine markings 

 on the Navicula Spencerii, since adopted as one of the most 

 difficult test objects. 



A communication from Dr. J. L. Smith, to Silliman's 

 Journal, describes the results of an examination of three micro- 

 scopes by different makers. From this it would seem that for 

 high powers, the object-glasses of Ross are the best, Spencer's 

 rank next, while Nachet' s are not much inferior. 



The best defining object-glass I have yet seen is one I have 

 made by combining two of Oberhauser' s with one of Chevalier's, 

 so as to make a triple objective. With this the sets of markings 

 on the Navicula Angulata are beautifully seen by oblique light. 



Such are the practical difficulties attending the production of 

 such delicate instruments, that there must be a very great 

 difference in the glasses even of the same maker, so that before 

 purchasing an instrument, it is always best to examine it by 

 means of some of the test objects hereafter described. 



REFLECTING MICROSCOPES, 



In which the image was formed by a concave mirror instead 

 of a lens, are not now so much used as formerly. They are 

 generally complicated in structure, and are surpassed and 

 therefore superseded by the achromatic microscope. 



The following is a simple reflecting microscope, invented by 

 Mr. S. Gray, and may be of some interest from its singularity. 



