306 The Microscope. 



ter of the objective and the optical center of the ocular. These 

 points are, however, the most difficult to determine, and they vary 

 with each objective and each eye-piece. The same objections hold 

 good with any measurement which has for its lower extremity any 

 part of the objective. Uniformity in the length of the setting, and 

 the position of the lenses of objectives, is practically impossible. 

 The lower extremity of the tube ("d" in Prof. Gage's figure) is the 

 only lower fixed point, and is the point selected by all but a very 

 few opticians. 



For the upper point "c" and "c"' can be excluded, "a" and "b" 

 being the only points that are fixed and accessible, and the majority 

 of opticians include the parts between one of these points and "d" 

 in their measurement of tube-length. These points can be deter- 

 mined by the youngest student, and variations in objectives will not 

 affect the length. Professor Gage prefers the measurement "b" to 

 "d." This is, perhaps, the simplest, but is open to the objection that 

 different opticians use eye-pieces of different construction. Euro- 

 pean makers use the Continental pattern, in which the eye-lens is but 

 one or two mm. above the body, while Americans prefer the eye-piece 

 with neck, which brings the eye-lens 12 to 15 mm. above the body. 

 This, of course, increases the optical tube-lengtli just so much, and 

 it would be necessary f^r opticians to indicate on the objective 

 whether it was corrected for the Continental or the American ocular. 

 With the measurement '"a" to "d" each microscopist could easily 

 adapt his tube -length to suit eitha- style of ocular. 



"We can join Professor Gage also in his plea for "par-focal" 

 oculars. Their adoption would be another step in the development 

 of a uniformity in apparatus, which is of so great convenience to 

 busy workers, and which tends so much to harmonize the work of 

 various manufacturers. 



We believe that these subjects, so tersely brought forward by 

 Professor Gage, should be agitated until mamifacturers adopt them; 

 and to further this end we shall be glad to publish correspondence 

 fi'om all interested opticians and microscopists. 



W^e regret exceedingly to hear that the able Secretary of the 

 San Francisco IMicroscopical Society, Mr. A. H. Brechenfeld, has 

 tendered his resignation as an officer of that society. Mr. Brechen- 

 feld will hereafter reside in San Diego, where we trust his enthusiasm 

 and interest in microscopical matters will inspire others to the for- 

 mation of a society and that much valuable work may be done in 

 this line of research. 



