ZOOLOGI AND BOTANY, MICROSCOPY, ETC. 1031 



If the objective is unacljustable, a statement should also accompany it, giving 

 the thickness of cover-glass for which it was adjusted." 



On this paper the editor of ' The Microscope ' * writes as follows : — 



" Every microscopist will thank Prof. S. H. Gage for publicly calling 

 attention, in his article, read at the recent meeting of the American Society 

 of Microscopists, to the remarkable lack of uniformity which exists among 

 opticians in their standards of tube-length and in the parts which they 

 include in their computation of it. 



All who seek and desire accuracy in their objectives, understand that 

 they are corrected for a definite tube-length, and that perfect performance 

 is possible only when that tube-length is used. The lack of knowledge, 

 even among expert microscopists, of the exact length for which given 

 objectives are corrected, and the difficulty of measuring it from the hidden 

 points adopted by many makers, have led them frequently to disregard the 

 perfect accuracy which they should observe in adjusting their Microscopes, 

 and to be satisfied with an approximation to the proper tube-length. Text- 

 books and makers' catalogues, also, are almost silent in the matter, and 

 microscopists who use the Microscope in their every-day business, but who 

 give but little attention to the optical principles of its construction and 

 working, have remained in ignorance of any necessity for such an adjust- 

 ment. Prof. Gage's article, with its complete tables, brings the subject 

 forcibly to the mind of every microscopist, and makes clear the necessity of 

 the adoption by makers of a uniform tube-length, and of uniform and easily 

 accessible points between which to compute it. 



Prof. Gage, in his remarks, rather hesitated to ask opticians to change 

 their various standards to a common one. From conversations with several 

 opticians we have learned that there are no serious objections to such a 

 change, and we urge upon manufacturers that it be made. The committee 

 appointed by the American Society of Microscopists to investigate the 

 subject and report at the next meeting, may, if their judgment agree with 

 ours, accomplish much to this end. 



A tube-length of 254 mm. is generally spoken of as the standard, and 

 is adopted by the majority of opticians, and this, we believe, should be the 

 only one chosen. 



In determining the parts to be included in the measurement of tube- 

 length there is more opportunity for diverse views. The most scientific 

 measurement probably, would be between the optical centre of the objective 

 and the optical centre 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 h 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 determined by the youngest student, and 

 variations in objectives will not aftect the length. Prof. Gage prefers the 

 measurement h to d. This is, perhaps, the simplest, but is open to the 

 objection that different opticians use eye-pieces of different construction. 

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

 1 or 2 mm. above the body, while Americans prefer the eye-piece with 



* Microscope, vii. (1S87) pp. 305-6. 



