RYLANDS, ON THE MICROSCOPE. 31 
1 shall not stay here to point out the advantages of obtain- 
ing the amount of penetrating power in the manner de- 
scribed ; this, and all that need be said further on the sub- 
ject, will, I trust, be sufficiently clear from what follows. 
The third power—the visual power of microscopes—is one 
which has been so rarely recognised as distinct, that probably 
even the name will be new to most of your readers. 
It is well known that the extent to which vision is aided 
by a telescope (for we must be mdebted once more to that 
instrument) is very rarely expressed by its magnifying power ; 
that two instruments, equal in both magnifying and defining 
power, may differ widely in their visual power; and as in the 
telescope, so in the microscope, for they are essentially the 
same in principle. 
Perhaps an example will most easily explain what is meant 
by visual power, and its connexion with the two already 
described. 
Some years ago, when my attention was first directed to 
this subject, I made the following experiment with a common 
marine “day and night glass.’ Having extemporised a 
““pancratic tube,” by which the power of the instrument was 
increased to 43, I directed it to a sign-board at the distance 
of 489 yards. This object had the double advantage of being 
readily approachable in a direct lune, and of having upon it 
letters of various sizes, so that it exhibited several degrees of 
legibility. Its distance, too, was ascertainable with sufficient 
exactness. Having impressed upon my mind the appearance 
of the board as presented by the telescope, I approached it 
until it was as legible and looked the same to the naked eye. 
From the peculiarity of the object, this point was ascertained 
at once within the limit of three or four feet. According to 
the popular idea, I ought to have been at one forty-third the 
original distance, the power of the glass being 48. Instead 
of this, however, I had passed over only fifteen sixteenths of 
the space; that is, the visual power was only 16, although 
the magnifying power was 43. This was not quite what L 
expected, but the examination was not long delayed. 
In order that an object shall be seen through a telescope 
(or a microscope) as when viewed at one forty-third the 
distance, it is necessary, not only that the angle subtended 
by it at the eye—the magnifying power—but also the angle 
subtended by the eye at the object—the penetrating power— 
shall be increased forty-three-fold. When this is the case, 
the visual power will be forty-three also. If we approach an 
object bodily, these angles naturally increase in the same 
proportion, but it is not so where optical instruments are 
