1877.] Microscopy. 53 
at Polaris Bay, July 5, 1872. The Arachnida were represented by four 
species, two of which have been identified by Mr. J. H. Emerton. Æri- 
gone psychrophila Thorell occurred at Polaris Bay June 3, 1872, and 
there were two unnamed species from Polaris Bay. At Foulke Fiord 
Lycosa glacialis Thorell was collected. All the spiders have been sent 
to Dr. Thorell to report upon. Upon the body of a Bombus Kirbyellus 
occurred several specimens of a Gamasus. 
Of fresh-water crustacea, besides a Copepod, Daphnia rectispina 
Kroyer occurred abundantly at Polaris Bay, August 1, 1872, as well as 
Branchinecta groenlandica Verrill. — A. S. PACKARD, JR. 
MICROSCOPY.! 
ILLUMINATION IN CONNECTION WITH POLARIZATION. — Mr. W. K. 
Bridgman, in a paper read before the Queckett Club, urges the impor- 
tance of polarization as an element of microscopical illumination. His 
thoughtful and suggestive paper is very interesting, though far from 
Gonelasivo in its demonstrations, and greatly marred by an apparent 
confusion in respect to the propagation, diffusion, and sensation of 
light. The essential point of his theory is that “it is the polarity in- 
‘duced by the impact of the ray, which excites or confers upon the re- 
flected or refracted portion of the ray a condition enabling it to convey 
the impression of the object to the eye, and the desideratum is to restrict 
the effect as much as possible to this one action.” It is not polarized 
light, but the act of polarization to which he attributes the effect. The 
excellence, for microscopical illumination, of light from a white cloud 
Opposite the sun he attributes to its entire freedom from polarization, 
while the inferiority of light from a direction at right angles to this, or 
from the blue sky, is attributed to its being strongly polarized, a scat- 
tered polarization being said to afford the worst possible kind of illumina- 
tion for the microscope. The author finds the best angle at which the 
light can be allowed to fall upon a painted surface to be its polarizing 
angle of about 56°, and to an approximate coincidence between the 
angle most conveniently obtained and the polarizing angle he ascribes 
the easy and general success in obtaining a good illumination of opaque 
objects by means of the side reflector or the condensing lens ; though the 
result is at least equally well explained, in both cases, on the old theory 
that this is the angle at which an intense illumination is easily obtained 
without the view being obscured by a glare of reflected light. In the 
use of the Licberkuhin; the author attributes the facility with which 
delicate details of structure ‘are rendered visible by light from one edge, 
oe short and thick. Spring much longer than in Z. nivalis, but shorter than in : 
or, not reaching to the insertion of the hind legs, while the fork is as long as 
the basal joint. It is black, and 0.06 inch in length. It differs from any American 
or North European species, and is certainly not the Podura humicola a Fabricius. 
1 Conducted by Dr. R. H. Waxp, Troy, N. Y. 
