8 THE MICROSCOPE. 



I find that the limit of my paper is reached without having ex- 

 hausted the subject of which it treats, and the further consideration 

 thereof must be left to a future one. In my next paper, therefore, 

 I shall hope to finish this, and, at least, make a commencement on 

 mounting in fluids, a subject that I regard as far more important 

 than either balsam or dry mounts, and one in which practical hints, 

 the result of many years of experience, may prove of more value to 

 the beginner than anything I may have heretofore offered. 



It may not be amiss to say, in concluding the present paper, 

 that I have lately, after a long series of experiments, succeeded in 

 perfecting an attachment, applicable to any microscope, whereby 

 negative enlargements of all objects not requiring a greater power 

 than % of an inch, maybe readily and perfectly made by any one, 

 even totally unacquainted with photography, and from these po- 

 sitions printed for throwing upon the screen with a lantern; at an 

 infinitesimal cost of money and time. The whole process is per- 

 formed by the simple aid of an ordinary coal oil lamp, neither 

 Heliostat or any other costly form of illumination being required. 

 I shall hope to describe and illustrate this apparatus in an early 

 number of this magazine. 



SOMETHING ABOUT OBJECTIVES. 



BY PROF. ALLEN V. MOORE, M. D., PROFESSOR OF MICROSCOPY IN 

 CLEVELAND MEDICAL COLLEGE. 



THE objective is regarded as the most important part of the 

 microscope, and deservedly so, for upon its correct perform- 

 ance, depends the real value of the instrument. 



The lower power objective and those in which the angular 

 aperture is less than 75°, are generally provided with no adjust- 

 ment for correcting the aberrations produced by the varying thick- 

 ness of cover-glasses, but when the aperture exceeds 90" or 100", 

 some correction is needed, in order to make the objective perform 

 equally well over various covers. 



This adjustment is made in various ways, according to the 

 quality or maker of the objective. In most of the best objectives 

 it is produced by a rectilinear approximation of the lenses; the in- 

 ner systems moving back and forth, while the front lens remains 

 stationary. In some other first-class (and especially English) ob- 



