448 SUMMARY OF CUBEENT BESEAROHES BELATING TO 



" Half-inch objectives had been made with apertures of 80°. 

 Some authorities had declared that 40° was the highest aperture that 

 could be usefully employed with that focal length. He had obtained 

 one of the best examples of the 1/2 in. of 80°, and had made a careful 

 series of trials with it. He had applied diaphragms above the back 

 combination to cut down the aperture to 60° and 40° respectively, 

 and the results might be briefly told. Taking the proboscis of the 

 blow-fly and viewing it with the 1/2 in, diaphragmed down to 40° aper- 

 ture, and arranging the illumination in the most favourable manner, 

 he noted every detail of the picture, the sharpness and blackness of 

 the points of the bristles, the transparency and clearness and general 

 precision of the image ; then removing the diaphragm behind the lens, 

 he increased the aperture to 60°, and he found the image improved 

 in every way. Increasing the aperture to the fullest extent, 80°, gave 

 no advance upon the quality of image seen with 60° up to the 1 in. 

 eye-piece ; for this reason he concluded that 60° was the really useful 

 aperture for a 1/2 in., and gave as much resolving power as the eye 

 could well sustain with that combined power. No doubt the extra 

 20° would give the lens a higher resolving power with a stronger 

 eye-piece, but he thought that might be better obtained with a lens 

 of shorter focal length." 



Mr. Nelson gives * the following table of apertures for object- 

 glasses (with 1 in. eye-piece on a 10 in. tube), and says that " if ideal 

 perfection is to be reached, the values given in the above table must 

 be aimed at." 



In. N.A. 



3 



2 



1| 

 1 



2/3 



1/2 



4/10 



1/4 



1/5 



1/6 



•08, air angle 10 



•12, „ 15 



■17, „ 20 



•26, „ 30 



•39, „ 46 



•52, „ 63 



•65, „ 81 



1 ■ 04, „ water angle . . . . 103 



1^3, crown glass angle 117 



1 • 56, which has yet to be constructed. 



It will be seen that there is a wide divergence between Mr. 

 Nelson's and Prof. Abbe's figures. For instance, for N.A. • 65 Prof. 

 Abbe suggests an objective of 1/8 in. and Mr. Nelson a 4/10 in. 



Lastly, we may give Dr. W. B. Carpenter's views as expressed in 

 his latest publication on the subject.f 



" The 1/8 in. is (according to the writer's experience, which is 

 confirmed by the theoretical deductions of Prof. Abbe) the lowest 

 objective in which resolving power should be made the primary 

 qualification, — the 1/6, 1/5, 1/4, and 4/10 in. being specially suited 

 to kinds of biological work in which this is far less important than 

 focal depth and dioptric precision. This view is strengthened by 

 the very important consideration that the resolving power given by 



* Engl. Mech., sxxviii. (1883) pp. 367-8. 



t ' EncyclopEedia Britannica,' 9th ed., xvi. (1883) pp. 269-70. 



