H. J. Clark on the Microscope. 41 
; have made so great a leap as if from an Oberhaeuser to a Spencer. 
; Since that visit, and another one also, made last summer, when 
.  lobtained one of Mr. Spencer’s quarter-inch objectives, with an 
, angular aperture of one hundred and forty-five degrees, I have 
| from time to time made particular efforts to test the value of the 
, flat field and wide angle in the study of organized bodies. The 
7 
be 
[ 
results of my investigations at Canastota, and also since my 
return, I have embodied in this 
The relations of the  rkas 
If the Wagnerian vesicle was situated at the upper or lower side 
of the Purkinjean vesicle, it has often been next to impossible 
difficulty I have seen obviated by the decided, section-like pre- 
cision of the flat field, which at once revealed to the eye the 
_ exact and relative level of every vesicle or yolk-cell. 
was most forcibly reminded, not long ago, of the value of 
the wide angle of aperture, and the accompanying great amount 
of light, upon trying Spencer’s objective upon the stem of a 
_ well-known Hydroid, the Clava leptostyla, Ag. In the manu- 
_ seript of the forthcoming volume of Professor Agassiz’s ‘‘Con- 
_ tributions to the Natural History of the United States of Amer- 
ica,” the outer wall of this Hydroid, and of several others, I 
may say In passing, had been Soscribta as a structureless mem- 
brane; but what was my surprise, in my last attempt, to find 
that this wall was composed of a layer of polygonal cells, as 
‘distinct as any in the other parts of the animal, and even readily 
discernible in the more opaque parts, where the stem appeared 
like a simple black surface under the ordinary microscope. 
SECOND SERIES, Vor. XXVIII, No. 82.—JULY, 1859. 
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