184 THE MICROSCOPICAL NEWS. 
initial magnifying power of the 1/25 in. enabled the observer to 
find some hitherto unknown object, or portion of an object, more 
easily than with the 1/12 in.; but when once found its details of 
structure would be better made out with the 1/12 in. So far it 
had not been possible to construct a 1/25 in. as perfectly asa 1/12 in., 
nor with so high an aperture; hence it would rarely bear any eye- 
plece beyond the lowest. The 1/12 in., however, with proper 
manipulation, would bear the 1 in. eye-piece, and then reveal 
structure that could not be made out with 1/25’s, as hitherto con- 
structed. 
“ 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° aperture, and arranging the illumination in the most 
favourable manner, he noted every detail of the picture, the sharp- 
ness 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 com- 
bined 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 Io 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. ° 
CT eee ‘08, air angle eae te ie Io 
De ee WP es ape EO oe 15 
i Se ae Are BS 20 
re ee, “26, Bs 30 
Zk ig Wile 1: ey 46 
Ti Deets *52, 5 63 
AlTO? seek O55 mets 7 Ae? a 81 
DiAte Tt tes 1°04, a, water angle ws OS 
B/G eeeses 1°3, crown glassangle ... 117 
MK) Seacee 1°56, which has yet to be constructed. 
* Engl. Mech., xxxviii. (1883) pp. 367-8. J 
