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On the OrticaL Powers of the Microscope. 
By P. G. Rytanps, Esq. 
Tue period has not yet arrived when even all those who 
employ the microscope methodically, as a means of scientific 
investigation, possess an intelligent comprehension of the 
principles on which it is constructed and the nature of its 
powers as an optical instrument. There is a large region 
beyond mere manipulation, into which few apparently care to 
enter. The writers of our introductory treatises have been 
curiously imitative in dealing with this portion of their duty. 
They indulge us with nearly the same very elementary 
diagrams, refer us to Ross’s capital article “ Microscope,” in 
the ‘ Penny Cyclopedia,’ and then, with here and there only 
a trifling exception, leave the matter pretty much as they 
found it. Surely the time has arrived which calls for more 
than this ; when an optical treatise on the microscope, worthy 
of the name, is not only desired by the few but required for 
the many. In the meantime, until this boon be granted, 
your pages will continue to do good service by dealing with 
these matters, and, as heretofore, in such a manner as to 
secure to your readers a large store of information. 
I had hoped that some more able hand than mine would 
have undertaken the subject on which I now propose to occupy 
a portion of your space; but it has not been so, and I there- 
fore offer the following remarks on the optical powers of these 
instruments to your readers, without further introduction. 
The first power which I shall mention requires little 
remark. It is the one which has attracted the greatest share 
of attention, from being that which constitutes the most 
prominent characteristic of the microscope. I mean magni- 
fying power. For our present purpose it is sufficient to 
remind the reader that magnifying power has to do with size, 
and size only. It expresses simply the dimensions of the 
enlarged image presented to the eye of observers, as compared 
with the size of the natural object when viewed at the adopted 
standard distance, ten inches, from the eye. Or, in other 
words, it may be said to express the magnitude of the angle 
subtended by the enlarged image, at the eye, as compared 
with that subtended by the object itself under the circum- 
stances named. 
The second, or penetrating power, is a subject which 
cannot be dismissed so easily. The origin of the term will 
be found in the ‘ Phil. Trans.’ for 1800, in an article by Sir 
William Herschel, entitled, “On the Power of Penetrating 
