82 RYLANDS, ON THE MICROSCOPE. 
used. Still, visual power must be a compound of the other 
two, and calling the three powers M, P, and V respectively, 
from their initials, we ought to have, im all cases— 
V=vVMP 
To test the experiment just related by this, the value of 
P having been carefully determined at the time, we find 
M = 43, P = 6, and 
V=V43 x 6= 16:06 
The value of V, as obtained by measurement, was 16:3, 
which is as near as could be expected under the circum- 
stances, although every precaution was taken to ensure cor- 
rectness. Visual power is, therefore, essentially the power 
of a telescope. 
IT need not extend this already lengthy article to show 
how entirely all this is applicable to the microscope also. 
I do not say that the variation will be as great in that 
instrument as in the telescope, for the construction is not 
only more uniform,* but the peculiarities of microscopic 
vision confine the matter in one direction, at least within 
narrower limits; but I do say that the time is long gone by 
for the distinctions I have pointed out to be neglected, or for 
us to have important and valuable terms drifting to and fro 
im our literature without any fixed meaning, threatened with 
expulsion by those in high quarters, and defined by each 
succeeding writer according as it seems good in his own eyes. 
Neither should we suffer ourselves to be deceived by large 
numbers, expressing amplification, it may be, but failing to 
afford us their promised aid in our search after natural truth. 
Fortunately the discoveries of the past quarter of a century 
have led us in the right direction; what we seem now to 
require 1s simply a correct determination of the value of x in 
the foregoing formule; we shall then be able, with very little 
trouble, to estimate the visual powers of our instruments, 
and shall have our efforts systematically directed to the in- 
crease and perfection of that upon which their value mainly 
depends. 
* This is more especially true of the instruments by our best English 
makers. The relative value of others will probably appear in a strong light 
when they are submitted to the test of visual power. The following ap- 
proximate estimates, obtained from a French instrument, will not be with- 
out interest : 
1st combination, M = 400, V (highest estimate) 145. 
2d AS M = 540, V, cannot exceed 905. 
3d is M = 870, V, does not reach 320. 
