174 
CORRESPONDENCE. 
manipulator in ' advanced microscopy,' he has not yet attained to a 
very high standard. Nor do we think that his knowledge of the 
capacity of glasses of different powers is sufficient for him to fully 
appreciate the comparative merits of high and low powers. In a 
recent article in the ' Monthly Microscopical Journal ' he undertook 
to prove that high powers, as a gV^h and upwards, were valuable in 
that by their means the differences in the size of the red blood- 
corpuscles of man and the lower animals, by the greatly increased 
amplification rendering their comparison easy, could be so clearly 
made out that there need be no difficulty in discriminating one from 
the other. Now, as has been pointed out in previous numbers of the 
* Medical News,' all the advantages of increased magnitude can be 
obtained by means of amplifiers, in any lens of sufficient power to 
bring an object into view. To be more definite ; with a Jth, or at the 
most with a y^th, with a high angle of aperture, the utmost limit of 
defining power is attained, and, having reached that, it is a matter 
of indifference at which end of the microscopic tube amplification is 
made. A ^^th, with an A eye-piece, magnifies 1250 diameters ; a ^^^th, 
with deeper eye-pieces and other means of amplifying, can be made to 
magnify several thousand diameters, without material loss of sharpness 
of outline. Where, then, is the advantage of the latter over the 
former ? In fact, it has been proven that the capability of the highest 
powers is less than those that are lower. For instance. Dr. Woodward 
has proven that a Powell and Lealand's ^^^g^th will show more than 
their ^th." 
The Advantages of High and Low Powers— An American Diffi- 
culty.— The editor of the Philadelphia ' Medical Times ' thinks that 
a committee of several microscopists should be appointed to settle 
the matter in dispute between Dr. J. G. Eichardson and Prof. Hunt, 
of the Woman's Medical College, as to the comparative merits of the 
former's ^\j-th and the latter's y^^th — or the advantages of high over 
low powers. 
COEEESPONDENCE. 
Observation of Test Diatoms. 
To the Editor of the * Monthly Microscopical Journal* 
London, February 8, 1875. 
Sib, — It may interest students of the diatoms most difficult to re- 
solve, to note a few of these objects as recently shown to me by Mr. 
W. J. Hickie, and I shall confine myself to a brief announcement of 
the principal results. 
1. Stauroneis spicula; with a ^^^th immersion very little could be 
made out, but with a ^^ih. immersion objective the lines were dis- 
tinctly observable. This seemed to be a very delicate and useful test. 
