On a Proposed Standard Micrometer Eye-piece. 
265 
10 inches.* of object and image, or of the two micrometers. If an 
objective is seen to magnify at the standard distance 5, 10, or 20 
times, call it objective No. 5, 10, or 20, or 0 5 , O ;o , O 20 . 
8. It also renders practicable the method of naming eye-pieces 
according to their real magnifying power. Thus, if the micrometer 
eye-piece makes T Vth or T ^ths in the eye-piece micrometer cover 
1 inch on a rule at 10 inches from the naked eye, call it eye-piece 
No. 10, or E 10 . Then since in the compound microscope the 
whole power (M) is the power of the objective (O) x the power of 
the eye-piece (E), we have 
O x E = M, 
0 5 x E l0 = M 50, 
O 10 x E 10 = M 100, 
O 10 x E 20 .= M 200, 
O 40 x E 10 = M 400, 
O 40 x E ;o = M 800, &c., &c. 
Any two of these three (O, E, M) being given, the third is easily found. 
9. With this eye-piece there need be no disputes as to whether 
a certain objective be ^th or T Vth, or whether a T \th be or be not 
T Vth, since it shows us how much it magnifies ; and here “ seeing is 
believing.” 
It would also save Dr. Royston-Pigott and others the trouble 
of taking the glasses out of their eye-pieces ; placing a micrometer 
in the empty eye-piece and examining it with a 1-inch lens ;f and 
also Dr. J. J. Woodward the trouble of having to use a dark room, 
a screen, and direct sunlight, to get the magnifying power of his 
objectives, in order to calculate their solar or principal focal lengths.! 
I do not pretend to a totally new invention ; but only to a new 
adaptation of an already existing instrument whose many uses in 
this direction seem to have been lost sight of. 
If not out of place, I should like to say that though not indebted 
to Dr. Royston-Pigott for my eye-piece (which grew out of my own 
wants), I am very much indebted for various useful hints scattered 
* I prefer 10 inches ; and one reason is that, at 10 inches, the real magnifying 
power is just 2 (and a fraction which may almost always be neglected) less than 
the nominal magnifying power. 
Focus 1 in., Nominal power 10 times, Real power 8 times nearly. 
» i v » 20 „ „ 18 
’> "3 >i )> ii ii 28 „ 
ii To ii ii 100 ,, ,, 98 ,, 
and conversely the seen real power + 2 = nominal (within the before-mentioned 
fraction). 
f ‘ Quart. Jour. Mic. Sci.,’ July 1, 1872, p. 270. 
