CORRESPONDENCE. 
MicroscoprcaAL CENTENNIAL HixHIBITIon. 
To the Editor of the ‘ Monthly Microscopical Journal. 
7, WiemMorr STREET, CAVENDISH SQUARE, 
February 20, 1877. 
Sirn,—The January number of the ‘ Monthly Microscopical 
Journal’ contains a review of the microscopes in the Centennial 
Exhibition at Philadelphia, in which the writer, Dr. Ward, while 
giving a most favourable notice of our exhibit, and expressing himself 
in a handsome manner as to the style and finish of our instruments, 
makes a remark which we fear may possibly bear a wrong inter- 
pretation. Referring to our objectives on Wenham’s new formula, 
Dr. Ward states that they were “ understood to have been entered 
for competition and then permanently removed from the Exhibition 
and the country.” This would imply that they were taken away 
before being examined, but this was not the case, as the judges tested 
the whole series critically on several occasions, and it was not until 
they informed our representative that their examinations were com- 
pleted that the glasses were removed from the building. We may 
add, that the judge’s Report is now in our possession, and that it is 
highly commendatory to both our microscopes and new objectives. 
We are, Sir, yours obediently, 
Ross anp Oo. 
ON THE MEANS OF CENTRING OBJECTIVES AND Roratina 
STAGES. 
To the Editor of the * Monthly Microscopical Journal.’ 
February 28, 1877. 
Si1r,— All who work with high powers, and use high-angled achro- 
matic condensers, have, no doubt, felt the inconvenience of the con- 
centric rotating stage being eccentric, not only with different objectives, 
but even, very often, with the one to which it may have been adjusted; 
and they may also have observed that the achromatic condenser, not- 
withstanding that it may have been carefully centred, will sometimes 
become eccentric. 
These defects are, of course, much less apparent in the very best 
workmanship. 
The means of “accurately centring the stage to the highest power 
objective” was introduced in 1875,* and later in the same year {+ the 
“concentric rotating stage having rectangular mechanical adjust- 
ments” was brought before the Royal Microscopical Society. Both 
these plans are based on nearly the same principle, and the result 
aimed at is to render the rotation of the stage concentric with any 
* «Science-Gossip, September 1875, p. lxvii.; and ‘M. M. J.’ No, Ixxxi. 
September 1875 (cover). 
+ ‘M.M. J.’ No, Ixxxv. p. 54, January 1876. 
