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CORRESPONDENCE. 
Brighton Meeting of British Association.* 
To the Editor of the 1 Monthly Microscopical Journal.' 
Royal Pavilion, Brighton, July 19, 1872. 
Sir, — I have the sanction of the General Manager of the London, 
Brighton, and Soutli-Coast Railway, to state that he will he willing 
to extend to the members of your Society the same railway facilities 
as will then be given to the members of the British Association during 
their visit to Brighton in August next. 
In a day or two you shall hear further from me. 
I have the honour to he, Sir, 
Very faithfully yours, 
Walter W. Reeves, Esq., 
Assist. -Secretary, Royal Microscopical Society. 
James Beck, 
Secretary to Executive Committee. 
The Measurement of Covering Glass.! 
To the Editor of the 1 Monthly Microscopical Journal.' 
Rochefort-sur-Mer, June 7, 1872. 
Sir, — In an article inserted in the last May number of the 
‘ Monthly Microscopical Journal,’ and entitled “ Optical Curiosities of 
Literature,” the Rev. S. Leslie Brakey points out an error in a note of 
mine published in August, 1871, in your estimable Journal. That 
note relates to the thickness of covering glass and of objects, and 
to the manner of measuring them. I remarked that error myself very 
soon afterwards ; but as the measurement of thin glass and of objects 
was not the exclusive subject of the note, I did not think it necessary 
to make a correction of it, but left to each micrographist the care of 
detecting the error now pointed out by the Rev. L. Brakey, and which 
I admit in all humility. I admit in fact that the thickness of objects 
cannot be ascertained by a change of focus, owing to the law of the 
transport des images. The same is true respecting the thickness of 
thin glass, which cannot be measured by transparency, because the 
lower surface appears to rise about Jtli (?). This phenomenon occurs 
with every object placed under a thin plate, according to the indices 
of refraction of the thin glass, of the liquid, and of the object itself. 
* Mr. Reeves lias requested us to publish this letter, which we do with great 
pleasure. Mr. Beck deserves the thanks of Fellows of the Society. 
t Some points in this letter are not particularly clear, but the translator is 
by no means to blame. — Ed. ‘ M. M. J.’ 
