76 MEMORANDA. 
jective at the same distances, it is impossible to make the 
results with that addition agree, particularly with the lower 
powers, where the proportion of one to the whole is so much 
greater, and where all errors are as a minimum. 
It is necessary to have the same distance from the stage 
micrometer to the focus of the eye-lens in measuring the 
magnifying power with the camera, as between the two mi- 
crometers in measuring the power of the objective ; ten inches, 
when possible, being most convenient. The adjustment of 
the objective to be always at the same place. 
Table of objectives.—Distances of micrometers divided by 
the number of eyepiece micrometer equal to one of stage plus 
one.—Focal length—Magnifying power by multiplying num- 
ber by 5 (power of eyepiece).—Also magnifying power by 
camera at ten inches, with longest eyepiece—The adjustment 
ring in a line. 
Magnifying 
Focal length. No. x 5. power by 
camera. 
Be D— 10-in. 
P1197 ~ 412 . 406: 
No. + 1 = 83°5 
4 D— 10 
— = ‘j91 - 256 . 256° 
No. + 1 =52:2 
+ D— 10 
—— = 390 des : 123° 
No. + 1= 25°6 
1 D— 10 
—_- = 93 ° 47°75 48° 
No. + 1=10°75 
2 D—11 
— = 1:98 : 23°50 . 23°7 
Not Ls 5°7 
I also enclose a method of finding the angular aperture by 
daylight. If it has not been published, and is worth inser- 
tion, perhaps you will find room for it also. It appears to 
me to have the advantage of being able to see clearly the ex- 
treme limits of distinct definition of the objective, although it 
is difficult to use with some objectives of low power, which do 
not give a sharp edge to the field of view. The method is 
by using the objective as a diminishing telescope, the eye- 
glass to be a common pocket or watchmaker’s eye-lens, of 
two or three inches focal length, so placed behind the objec- 
tive as to show distinctly a rule or a wall a few inches or more 
