132 
CORRESPONDENCE. 
apertures, by mistaking or including angle of diffusion for angle of 
aperture, and Mr. Tolles wearies us by wishing to claim special 
exemption for himself. If he feels aggrieved by my counter-statements 
of his alleged apertures, or attributes a malicious motive to me in 
making them, why not object to my sole evidence ? and ask for a formal 
trial by others, with the slit in any way that he may suggest, I have 
no desire to interfere further, and as I wish the question off my hands, 
I will aid the trial by the loan of any arrangement that is needed. I 
append this, that Mr. Tolles may not have to wait till I am " heard 
from again." 
A Self-centering Turn-table. 
To the Editor of the ' Monthly Microscopical JournaV 
13, William Street, New York, Feb. 1, 1875. 
Dear Sir, — -Herewith I enclose two drawings illustrating an inven- 
tion of mine for the improvement of the turn-table now in use with 
microscopists. The particular purpose of the device is to obtain 
uniform centering of slides, without measurement or trouble of any 
kind. The drawings are made to a scale of one-half of the actual 
measurements of the table which I have, and which I lately exhibited 
to the American Microscopical Society of this city, of which Society 
I am the secretary. 
Fig. 1 is the upper face of the table, and Fig. 2 is a perpen- 
dicular section through the line of the milled-head and spindle. 
