INVERTED MICROSCOPE. 383 



as the twentieth of an inch; and I have two such in my pos- 

 session, made for me by Prof. Eiddell, and they are certainly 

 the most convenient things of the kind I have ever used. And 

 here I may remark that for all observation with high powers 

 the Inverted Microscope is decidedly superior to the ordinary 

 forms of mounting ; for in the latter case, when an object-glass 

 of a one-twelfth or one-sixteenth inch focus is used, the focus 

 is too short to admit of the use of cells ; whereas in the inverted 

 form, as the object is looked at from beneath, the cell may be 

 as thick as one pleases. Another thing that I have discovered 

 connected with this class of observations is that the Desmidice 

 and Diatomacice can be observed to much greater advantage 

 from beneath than from above, for reasons that will be obvious 

 to persons accustomed to observe these classes of objects. 



Another advantage possessed by this instrument, calculated 

 to extend its use for general purposes, is its great capacity for 

 every variety of illumination, without sacrificing the ease and 

 freedom from fatigue belonging to the use of this form of micro- 

 scope; for when placed on a table, rather higher than the one 

 commonly used, and a foot or two from the edge, the observer 

 Can recline on his arms, and observe for hours without the 

 slightest sensation of fatigue.* 



*As Prof. Eiddell, of the Medical Department of the University of 

 Louisiana, has been using my microscope for general purposes for more 

 than a year, I requested of him his opinion as to its advantuges, which is 

 expressed in the following letter: 



Prof. J. Lawrence Smith: Dear Sir. — In reply to your note respecting 

 your Inverted Microscope, I have to say that having formerly heen in the 

 habit of using the mountings of Pritchard, Dollard, Easpail, Chevalier, and 

 Nachet, and having the past year constantly used my best lenses (Spencer's 

 make) in the inverted microscope, I am fully satisfied of the practical 

 superiority of the latter for general purposes. With it observation can be 

 made with more ease and comfort, the light admits of more convenient and 

 efficient management, chemical re-agents can be applied to the object with 

 the greatest facility, without endangering the instrument, and the slides can 

 be moved or changed with the utmost facility, and with perfect safety to 

 the object-glass and the slides themselves. The instrument is so firm as to 

 manifest no vibration with the highest powers, and admits of the attachment 

 of every collateral appliance. I shall never willingly return to the habitual 

 use of any other known form of microscope, especially with high powers. 

 The excellence of your form of microscope depends on having a good 

 reflecting prism below the object-glass. The one used by me, made by 

 Oberhauser of Paris, seems to be perfection itself, and seems neither to. 

 absorb or distort the luminous rays in the slightest degree. 



Kespectfully yours, J. L. Eiddell. 



