174 THE MICROSCOPICAL NEWS, 
we have to consider is the amplification, and we shall find that 
a four-inch lens magnifying 10 diameters, with the A ocular, yields 
us a penetrating power from the eye alone of 2,080 micras ; a one- 
twentieth of an inch gives only a penetration of one-fifth of a micra. 
II. The “focal depth” of the objective depends upon the aper- 
ture employed for producing the various amplifications. A four- 
inch of 16° air-angle would have a focal depth of 262 micras, 
while an inch and-a-half objective, of the same air-angle, yielding 
an amplification of 30 diameters, could only penetrate through 86 
micras. 
A direct comparison of several objectives of different apertures 
and varying amplifications, may with advantage be introduced here, 
and the table goes to show that what is seen down the tube of the 
microscope results from the combined effects of the eye and the 
objective, but when a picture is thrown upon a sensitive plate it is 
evident that the first quantity is nearly eliminated, and the only 
penetration attainable is that which the lens itself possesses. ‘The 
objectives are each and all supposed to be used with the A ocular. 
TABLE I. 
‘ Accommo- 
Penetrating|Focal Depth| dation  |Total Depth 
Inch. Air Angle.| Aperture. Power in Depth of | of Vision 
| iL Micras. Eye in in Air. 
| a Micras. Micras. 
4 3° 07 14.30 522 2080 2602 
4 16° nA 7.19 262 2080 2342 
1% 16° 14 eit 86 230 316 
1% 20° 77 5.75 69 230 299 
1% 24° Att 4.81 57 230 287 
% 40° 34 2.92 10.6 20 30.6 
% 70° Ce 1.74 6.3 20 26.3 
%| 110 82 r22, 4.4 20 24.4 
| Fa. 60 1.66 1.99 2.3 4.29 
| 100° 76 int TS 2:3 3.87 
Sy oaanet vee 1.20 83 .99 2:8 3.29 
1 TIO” 83 1.20 Te 58 1.30 
az | 144 97 1.02 61 58 1.19 
1 : Lalo OI 54 58 be 
sy | 160 98 1.02 37, et 58 
Se imeoeece I.10 Voy .33 21 54 
From the foregoing table it will be seen that large objects cannot 
possibly be penetrated even with objectives of low angle and 
medium power. The seeds of the Se¢w/a alba measure 1,100 
micras across them, and require therefore 550 micras of penetration 
