28 RYLANDS, ON THE MICROSCOPE. 
into Space possessed by Telescopes.’ In that article we are 
told that when, owing to the darkness, a distant church- 
steeple was invisible, a certain telescope described showed the 
time by the clock upon it very clearly. This, adds Sir 
William, was not owing to magnifymg power alone, for the 
steeple could not be discerned by the naked eye. 
Following out the suggestions of this incident in a truly 
philosophic spirit, the author has given us, in the article 
referred to, all that is required to apply the term correctly to 
the microscope. 
Unless I am mistaken, the first use of the word in con- 
nexion with the microscope occurs in the ‘ Microscopic 
Cabinet.’ Judging from the manner in which it is there 
employed, we should perhaps define it as synonymous with 
angular aperture. Most persons, I fancy, were at a loss to 
see the connexion between the name and the thing signified, 
for, while some few writers were content to adopt the term 
with the explanation given, others, considering it an entire 
misnomer, began to speak of angle of aperture, and have 
since defined ‘“‘penetrating power” to mean superior definition, 
thickness of field, &c. This has naturally led to confusion, 
and that not amongst those only who make small pretensions. 
Dr. Carpenter, in his ‘ Manual,’ tells us that the penetrating 
power of an olject-glass “depends upon the degree of dis- 
tinctness with which parts of the object that are a little out 
of focus can be discerned,” or, in other words, longitudinal 
focal range or thickness of field. The editors of the ‘ Micro- 
graphic Dictionary’ mention “two distinct kinds of pene- 
trating power,” one the same as defining power, and the other 
angular aperture, combined with oblique illumination. They 
propose that the term should be laid aside as tending to con- 
fusion! I think it may be shown that the confusion is not 
altogether attributable to the term, and that the whole 
difficulty not only admits of an easy solution, but that the 
subject is sufficiently important to warrant a careful investi- 
gation. 
The authors of the ‘ Microscopic Cabinet’ had in their 
minds, there is no doubt, the true origin and meaning of the 
term. They erred in not giving a sufficient explanation. 
They borrowed it from the telescope, and, seeing that the 
telescope and the microscope are essentially the same instru- 
ment, but modified to adapt them to different purposes, the 
use they made of it was perfectly justifiable; at the same 
time it must plainly be used to mean the same thing in both 
cases. Sir William Herschel has shown, in the article 
already referred to, that the words penetrating power have a 
