ADJUSTABLE OBJECTIVES. 149 



Here we have an argument in favor of wide angles 

 which has been in the past quite lost sight of, and was 

 for the first time presented to the public by the writer 

 in one of his monthly contributions years ago ; but all 

 this is foreign to our present purpose. 



Now, in the case of the one-sixth of Mr. Tolles, 

 should it be attempted to use this glass over the thin 

 cover of the Moller probbe plate, the result would only 

 be to defeat the performance, for the glass would "cor- 

 rect" within three divisions of "open-point." But, 

 per contrary, if we desire to secure the best definition 

 of the said objective, and over the said plate, it becomes 

 necessary to supplement by thicker cover (making sure 

 of optical contact by the use of water or glycerine), 

 until the glass shall " correct" at or near " closed." 



Hence, it occurs that it is essential that the observer 

 who proposes to employ objectives of wide apertures 

 should pay some attention to the condition of things ; 

 that he should know precisely over what covers he will 

 be enabled to get the maximum performance. In 

 short, to ascertain where he may find the maximum 

 aperture of his objective. 



And to this end, we propose to aid and assist, as far 

 as possible, in some future remarks we shall have occa- 

 sion to make regarding the manipulations of the objec- 

 tive. During the past twelve years the author has 

 received hundreds of letters from as many individuals 

 commencing the study of the microscope, desiring such 

 information as to the best way of making an invest- 

 ment as may pertain to the selection of stand, objec- 



