450 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



" It must be understood that there is no intention in these remarks 

 to undervalue the efforts which have been perseveringly made by the 

 ablest constructors of microscopic objectives in the direction of 

 enlargement of aperture. For these efforts, besides increasing the 

 resolving-power of the instrument, have done the great service of pro- 

 ducing a vast improvement in the quality of those objectives of mode- 

 rate aperture which are most valuable to the scientific biologist ; and the 

 microscopist who wishes bis armamentum to be complete will provide 

 himself with objectives of those different qualities as well as different 

 powers which shall best suit his particular requirements." 



"High-angled" Objectives. f — Dr. J. Edwards Smith "prefers 

 to regard as 'high-angled,' any, and all glasses, vdthout reference to 

 their focal lengths, which are endowed with the widest apertures 

 obtainable. If this be accepted, then it will occur that a 1 in. 

 of 50° should be classed as a high-angled objective, and similarly 

 a 2 in. of 25°. And, again, it would also then occur that a 1/6 in. of 

 130°, which fifteen'years ago ranked as a wide, would now be classed as 

 a glass of medium power." 



Zeiss's A* Variable Objective and "Optical Tube-Length." — 



The demonstration of the important influence of " optical tube-length " 

 on the magnifying povs^er of the Microscope explains what has hitherto 

 seemed a curious anomaly in the action of this objective. 



It will be remembered that it has a considerable range of power 

 according as its two lenses are " closed " (when they are 44 mm. apart) 

 or "open" (when they are at a distance of 52 mm.), the closing and 

 opening being effected by rotating the collar on the objective. 



In the closed position the equivalent focal length of the objective 

 is 54 "1 mm,, and in the open 39*7 mm., or a ratio of approximately 

 4 : 3. The power of the Microscope is however increased not in the 

 ratio of 3 : 4 only, but of 3 : 5 • 28. 



The explanation of this difference is found in the fact that A, or 

 the optical tube-length, varies considerably according to the position of 

 the lenses of the objective. When they are closed the posterior focal 

 plane is 153 • 6 mm. from the back lens of the objective, but when open 

 125-7 mm. only. A is therefore (with a tube-length of 10 in. or 

 250 mm. from the back lens of the objective to the anterior focal plane 

 ofthe ocular) 250- 153-6 = 96 -4 mm., or 250 - 125-7 = 124 -3 mm. 



t 'How to see with the Microscope,' 1880, p. 104. Of. also p. 146. 



