ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 
121 
Hence, for the emmetropic observer whose accommodation is entirely 
relaxed, or for any observer whose eye is accommodated for parallel 
rays, the total amplification of the 1-in. objective and the 2-in. ocular 
will be 9 x 5 = 45. 
For strict accuracy, the change in the position of the image produced 
by the objective, if the adjustment for the different eyes be produced in 
the usual way by means of the fine or coarse adjustment moving the 
objective to or from the object, should be taken into consideration, but 
the amount is so small, about 2^ per cent, in the first case, and 5 per 
cent, in the second, that it may be neglected without seriously impairing 
the practical accuracy of the general result, while if the adjustment for 
different eyes be made with the draw-tube moving the ocular only, the 
position of the image produced by the objective is not changed, and 
therefore, so far as it is concerned, the original formula remains strictly 
correct.” 
“ Table A. 
“ Amplification (linear) of Real Images projected at 10 in. from optical 
centre by simple bi-convex lenses. 
Focal Length of 
Linear Amplifi- 
! Focal Length of 
Linear Amplifi- 
Lens in inches. 
cation of Image. 
Lens in inches. 
cation of Image. 
5 
1 
1/4 
39 
4 
1-5 
1/5 
49 
3 
2-33 
1/6 
59 
2 
4 
1/7 
69 
1 
9 
1/8 
79 
3/4 
12*33 
1/9 
89 
2/3 
14 
1/10 
99 
1/2 
19 
1/12 
119 
4/10 
24 
1/16 
159 
1/3 
29 
1/25 
249 
Table B. 
“ Amplification (linear) of Virtual Images projected at 10 in. from optical 
centre by simple bi-convex lenses. 
Focal Length of 
Lens in inches. 
5 
4 
3 
2 
Linear Amplifi- 
cation of Image. 
3 
3- 5 
4- 33 
6 
7-73 
11 
Focal Length of 
Lens in inches. 
3/4 
1/2 
4/10 
Linear Amplifi- 
cation of Image. 
14 
21 
26 
31 
41 
“ Note. — In the Huyghenian ocular (the form most commonly in use) the field- 
lens, while mechanically part of the ocular, is optically part of the objective, in that 
it contributes to the formation, not of the virtual image projected by the ocular, but of 
the real image projected by the objective, upon which it acts negatively, diminish- 
ing its size while increasing the superficial area brought into view at one time. So 
that, in this form of ocular it is the eye-lens alone that contributes to the reamplifi- 
cation of the image, but the negative action of the field-lens must, of course, always 
be taken into consideration when attempting to determine the amplifying power of 
a Huyghenian ocular by calculation.” 
