310 General Notes. [ April, 
meter, light may be sent through them from an aperture of known 
diameter, and a piece of card or ground glass placed between the 
lenses at different points to indicate the course of the rays. For 
measurements, one of J. Moller’s photographed micrometers, in- 
serted as an object, is most convenient, the dense black blocks 
admitting of accurate reading, and the millimeter being a more 
convenient unit than the line. The magnifying power is ascer- 
tained according to the ordinary formula: Divide the product of 
the focal lengths by their sum diminished by the interval between 
them; e. g., for the first in the table 30x60=1800, which, divided 
by 30+60—58=48, the result being slightly too large. 
| v gs] | fa , | 3 v a 
| o pae] e egeta Ei S| 8 : 
| Ba thd asl a atl AlE dy 
| 5 s | na Ot e Geet A a A a | 
| salgyt el 3 Agagi E Elig] Pise 
Maker and Name. | %5 mel S E | Sap] 5 £\|lga| $ PE 4 
| Ca c ee ae wet ee ee) Se aes 
| -i o o |é E E E e e . z 
| ae tae BG ee ee Coe Ee We Lae aioe - 
Big ALAA A) t aie | eA 
Billo p i te A| 30 | 60 | £8) 29°) 53°) 34 1718) 74 | 482s 
2 | Popular. Bia Aal i ha oe) oo twee (33. eS 
34 Cite] a0) 9 94 ae | ta) toe E] | 20 |... $ ERS 
4 | AN se) 60. 8 F 40} Se aa 7 88 Pee Be Relies | 
5 | =. | Bl 20 40 t1} 24 | 38 | 20:15} 7 FF 136) r} 6 
6 De C 14 |. 25:| 7 | 134] 24 | 18) 101-5 |.% | 23 ae 
7 ga D: 8) 96-16) 412) 48) 1g} Baby any 8 S 
8 g El 6) 92455 on ake 8 oo ly abe 10 | $129 
9 Bho PS Os S| T 2}. 947 $13 
tö | Orthoscopic i2 2s 9 | 17 | 25 | 17112) 8. 251 |10 
11 f | 96'} 3§-}-a194] gr | 37) | FE 1 38s S 
12 11 30| 55 14| 20 | 47 | 30 | 13 itr | 43) 14| 5 
13 | Nachet. 2.1 20 | 38.1.13 | 20 | 35 |.18 | 13 i+] 33, 1417 
4 ELE ie | 30 | 37 | 10 E p26 L 119 
I ec te on. I 28 | 52 20 | 10 50 | 2 + 
1 et sie Praes 24 37 eae 8 if 37 1:24: | © 
17 2 |32| 43 | I2 | 19 | 47 | 35 | 14 49 2 | 4 
18 Oberhauser. { 3| 29 | 37 | 10 | 20 | 40 | 30 | 15 x 41; 18} 5 
(All distances in millimeters except in the column marked “ inches.’’) 
By inspection of the table it will be seen that in half the oculars 
examined the ratio of focal length of eye lens to field lens is about 
one-half, in only one is it one-third, and in one of older construc- 
tion they approach so near as seven-ninths. The general prin- 
ciple in regard to the interval separating the lenses is that it 
shall be less than the solar focus of the field lens though in the 
deeper oculars, and in the orthoscopic, this limit is approached 
or slightly exceeded. But it must be remembered that in com- 
bination with the objective the ocular receives diverging rays, and 
hence the actual focus of the field lens for such rays is beyond 
the eye lens. In the shallow oculars it will be seen that only the 
central portion of the eye lens is used.— W. H. Seaman (abstract) — 
in Nat. Mic. Congress. 
? 
