1884.] 



MICKOSCOPICAL JOURNAL. 



136 



sometimes happened that the high in- 

 itial magnifying power of the gi^ en- 

 abled the observer to find some hith- 

 erto unknown object, or portion of an 

 object, more easily than with the ^ ; 

 but when once found its details of 

 structure would be better made out 

 with the ^V. So far it had not been 

 possible to construct a ^g^ as perfectly 

 as a ^, nor with so high an aperture ; 

 hence it would rarely bear any eye- 

 piece beyond the lowest. The ^, 

 however, with proper manipulation, 

 would bear the i-inch eye-piece, and 

 then reveal structure that could not be 

 made out with ^igS as hitherto con- 

 structed. Half-inch objectives had 

 been made with apertures of 80°. 

 Some authoi'ities 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 ^ inch of 

 80°, and had made a careful series of 

 trials with it. He had applied dia- 

 phragms 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 ^ inch diaphragmed down to 40° 

 aperture, and arranging the illumina- 

 tion in the most favorable 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 ap- 

 erture to 60°, and he found the im- 

 age improved in every way. Increas- 

 ing the aperture to the fullest extent, 

 80°, gave no advance upon the qual- 

 ity of the image seen with 60° up to 

 the I -inch eye-piece; for this reason 

 he concluded that 60° was the really 

 useful aperture for a half inch, and 

 gave as much resolving power as the 

 eye could well sustain with that com- 

 bined power. No doubt the extra 

 20° would give the lens a higher re- 

 solving power with a stronger eye- 

 piece, but he thought that might be 

 better obtained with a lens of shorter 



focal length. In the matter of eye- 

 pieces, so far as he had not done use- 

 ful work with anything higher than 

 the one inch, except in adjusting the 

 correction collar in testing the object- 

 ives. With reference to the question 

 of the relation of aperture to power, 

 he alluded to a note ahx'ady commu- 

 nicated to the club in which he had 

 said that by experiment he found that 

 the normal eye was capable of de- 

 fining objects at a distance of 10 inch, 

 making an angle of i' 23'^ — that is to 

 say, it was capable of separating 

 lines ruled at the rate of 250 to the 

 inch. Applying this to microscopi- 

 cal vision with a half inch the ques- 

 tion was. What aperture was neces- 

 sary to enable it to resolve anything 

 that would be brought to that visual 

 angle by an amplification of ten times 

 its initial power ? For example : the 

 half inch having an initial power of 

 twenty, with a i-inch eye-piece would 

 magnify 200 diameters ; multiplying 

 the last figure by 250, we obtained 

 the product 50,000 ; an aperture of 

 63° would be required for the resolu- 

 tion of lines of that fineness. He 

 might warn microscopists generally 

 of the immense number of grossly de- 

 fective eye-pieces that were oflered 

 for sale. The Huyghenian eye-piece 

 should be constructed so that the 

 radius of curvature of eye-glass to the 

 field-glass should be as 3 : i . Certain 

 manufacturing opticians had entirely 

 lost sight of the formula, and they 

 made the same field-glass do duty for 

 eye-glasses of difterent foci. The re- 

 sult was lamentable. Well-made eye- 

 pieces were most essential to good 

 wodc with the microscope. Here, 

 again, he could bear testimony to the 

 conscientious accuracy of Messrs. 

 Powell and Lealand, who had not 

 departed from the strict Huyghenian 

 formula for eye-pieces. On the use 

 of daylight he said he found it effect- 

 ive for low powers up to f inch, and 

 with condenser up to ^ inch. The 

 direct sunlight involved the use of a 

 heliostat, otherwise the continued ad- 

 justment of the mirror was irksome. 



