1876. ] Microscopy. 443 
the greatest thickness of glass cover which it would work through as 
.018", and I will supply the triangle, 
(Figure 26) to suit the new distance 
in glass, 
Measured by the outside lines, we 
have here a balsam angle of 100°. 
But the whole opening of the front 
face of an objective of quite moderate 
power, even, is seldom used. Accord- 
ingly, nearly a year of interval again ae 
having elapsed, he gives the utilized aperture of the front lens, front sur- 
face, as 083’. The triangle thus be- 
comes as the dotted lines make it in 
as can be practically “considered, and 
the difference between 82° and 88° 
clearly an increase for balsam r Fre. 27. 
mounts over what a dry lens can have. 
The fact is that the ray emerging into air at the extremest distance 
from the axis for the in-air utilized area, hugs the surface of the lens, 
traversing the air parallel to it. — R. B. ToLLEs, Boston, May 5, 1876. 
OXALATE or Asparagine. — Mr. C. C. Merriman’s slides are pre- 
pared from saturated solutions of asparagine and oxalic acid, and solution 
of gum-arabic, about in the proportions of five, four, and three parts of 
each respectively. The solutions are mixed only in small quantities for 
immediate use, and the proportion varied according to the effect pro- 
duced. When dry the specimens are protected by a thin film of collo- 
dion before mounting in old balsam. 
Action or Porson on BLoop. — Dr. Blake found that one grain of 
sulphate of thorium injected into the blood vessels of a rabbit caused 
eath in two minutes, after which the blood-corpuscles, having entirely 
lost their natural form, presented an indented outline with numerous 
highly refracting dots at the circumference. 
Excuancrs, — [Notices not exceeding four lines in length, of micro- 
Scopical objects, or apparatus wanted or offered in exchange, not sale, 
Will be inserted in this column without expense. 
Seeds of Paulownia imperialis, in exchange for other good objects. 
—H. S. Moore, Sixth Avenue, corner 43d Street, New York. 
A large variety of objects, in exchange for any good slides. Lists 
furnished on application. — W. G. CorTHELL, 103 Warren Avenue, 
Boston, Mass. 
Well mounted and named slides wanted in exchange for cabinet-size 
Photographs of the objects. — R. H. Birven, Elmore, Ohio. 
