( 130 ) 
COKKESPONDENCE. 
The Formula of the " Museum " yV^H. 
To the Editor of the Monthly Microscopical Journal* 
Boston, January 21, 1875. 
SiRj^Yon have published a diagram of a Ytth inch objective de- 
scribed by Dr. Woodward in the number of your J ournal for November, 
1873, p. 214. The diagram appears in connection with a compu- 
tation of the angular aperture of the objective by Prof. E. Keith 
given in this Journal for September, 1874, p. 124. 
In your Journal for Jime, 1872, p. 273, I find that Mr. Wenham 
says this, — " If Mr. ToUes will give us the correct radii, diameters, 
distances, &c., of any one combination that has been actually con- 
structed and transmitting a full aperture in balsam, and show us 
the passage of the rays through to prove the result, then I will strike 
my colours ; or if he will furnish the drawing, the rays shall be 
traced through for him." 
The same suggestion substantially was made to Dr. Woodward, as 
quoted by Dr. W. in his last article, this Journal for September, 
1874, p. 125. 
In view of these invitations and proffers on the part of Mr. Wen- 
ham, I have furnished the formula on which the " Museum " yo^th in., 
measured and computed, was constructed. It was carefully recorded 
from actual measurements of the curves, diameters, and distances. 
There can only be a question of care and accuracy in taking these 
dimensions. In support thereto I will state that it has been my 
custom for a decade or two to record such dimensions for my own use 
and convenience, in the case of all objectives of any considerable 
merits constructed under my direction. I also send you the dimen- 
sions of the next -joth. made on the " Museum " th in. formula, and 
which was fully tested at the Museum by Dr. Woodward, and endorsed 
as similar to the " Museum " y^th in. in its traits. 
For convenience of comparison I give the formula in the same 
tabulated form adopted by Prof. Keith. There are but slight varia- 
tions as to the back and middle systems, while the front is identical. 
a. 
6. 
c. 
d.. 
e. 
/. 
£>■ 
Eefractive index . . 
1-525 
1-620 
1-525 
1-620 
1-525 
1-654 
1-525 
Rfidius of 1st surface 
2nd „ 
Thickness at centre 
0-265 
0-200 
0-050 
0-205 
0-200 
GO 
0-020 
0-205 
CO 
0-200 
0-040 
0-205 
0-200 
0-510 
0-027 
0-205 
0-100 
0-180 
0-067 
0-167 
0-180 
0-500 
0-010 
0-167 
0-033 
CO 
0-035 
0-066 
Yours respectfully, 
EOBT. B. TOLLES. 
