ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
543 
my own construction, and to include the results of this paper, but the 
pressure of professional duties has prevented the completion of this work 
in time for this meeting. A description of this piece of apparatus may, 
however, not be inappropriate. See fig. 80. 
Fig. 80. 
Fig. 81. 
A represents an iron plate, 20 in. square and 1/2 in. in thickness, 
planed as flat as possible on its upper surface; B represents a brass 
circle 13 in. in diameter, graduated to one-half degrees, and turning 
around its centre C; D represent two opposite verniers, reading to 
single minutes of arc. 
In practice, however, these are entirely unnecessary, as the unavoid- 
able errors of measurement must exceed the least count of the verniers. 
The centre is read, therefore, simply to one-half degrees or, if desired, 
by estimation to three minutes of arc. 
In fig. 81, A represents a brass slide with a small hemispherical lens 
B, burnished into an opening in its centre. The centre of this lens is indi- 
cated by a very small circle marked on its plane surface with a diamond 
while in the chuck on which it was turned up. This portion of the 
apparatus was made for me by Spencer and Smith, of Buffalo, New York. 
