The Microscope. 305 



EDITORIAL. 



UNIFORMITY OF TUBE-LENGTH. 



T7VERY microscopist will thank Professor Simon H. Gage for 

 -■--' publicly calling attention, in his ai'ticle, read at the recent 

 meeting of the American Society of Microscopists, and published in 

 this number of The Microscope, 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 

 adjustment. Professor 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. 



Professor Gage, in his remarks, rather hesitated to ask opticians 

 to change their various standards to a common one. From conver- 

 sations 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 A. S. M. 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 cen- 



