134 THE MICROSCOPICAL NEWS. 
nl 
No. 2 Z | (cane 
Hore tre 10.4 10 | .80 
2 peat ne 10.9 IO | 17 
a page 12.0 fe) fi 
Aeneas 9.0 10 go 
Cheat | 9.5 10 86 
uae caer 10.0 10 82 
The student may have good practice in “this process by varying 
the distance between the two micrometers, to the utmost limits of 
his instrument; he will find it excellent practice, and as a result 
he will be aware of the exact nature of his objectives, and better 
able to follow us in our remarks in subsequent chapters. 
The formula here given is usually termed Cross’s formula, and is 
a very good method for observers at a distance from each other of 
comparing the magnifying power of their objectives. It has been 
suggested that the method is not absolutely accurate, but it is 
sufficiently near for all practical purposes. 
Eye-pieces may be measured in exactly the same manner; the 
student will be able with a little ingenuity to rig up a small box 
with an aperture at one end to receive the eye-piece, and a ground 
glass at the other, ruled with pencil into inches and tenths ; this, of 
course, takes the place of the eye-piece micrometer in the former 
example. The eye lens of the ocular is made to point towards a 
rule, divided into inches and tenths, and this rule is moved back- 
wards or forwards until a correct focus is obtained. The magni- 
fying power can be then easily reckoned, and when the distance 
between the two scales is known, the calculation is just the same 
as for objectives. The following measurements were made by the 
Editor of the oculars of some of our leading firms :— 
DESIGNATION. | n Z a2 - 
(z+1)? 
PA nta2 abe qaspiaels ie) 25 2.06 
Be accwieaicpm sere ce 16 sas 1.38 
Ramsdenm's Boric. .temtene a 12 18 1.28 
Periscopic one inch....... c7 19 1.00 
ehh Oe Sore eee 25 24.5 -94 
ReemetCynsecec ose 26.5 24.5 85 
Direc rete 45 25 ae 
| DP een eee 57 24.5 41 
| Rees seriegs eee ome 6 80 24.5 .30 
Molles @sOud: <9. s)-uans 250 20 ‘2 
