442 MICROSCOPY. 
the objective its own illuminator; but instead of the silvered 
reflector, thin glass disc, or reflecting prism, used by Prof. Smith 
and subsequent experimenters, he employs the binocular prism of 
Wenham’s binocular. A beam of light reflected down the oblique 
body of the Wenham binocular, the prism being in position as for 
ordinary use, is of course condensed by half the objective upon 
the object in its focus; while the other half of the objective and 
direct body of the instrument is used at the same time as a monot- 
ular for viewing the object. A portion of the field is intensely 
. illuminated, and definition is said to be better than with other 
arrangements. 
Cotiins’ Licut Corrector.— Mr. Collins, of London, has intro- 
duced a modification of Rainey’s Light Modifier, to correct the 
glare of too intense light or the yellowness of gas-light, lamp-light, 
ete. A thin plate contains a rotating wheel with four apertures 
one of which is empty while the others present in turn a ground 
glass and two shades of blue glass. This contrivance, an illustra- 
tion of which may be seen in the advertisements in ‘+ Science nia 
sip,” is always ready for use, being simply laid, under the object, 
upon the stage of any microscope ; though for use with an achro- 
matic condenser it is somewhat better to have the correcting plate, 
whether of ground or blue glass, below the condenser. : 
form of Light Corrector may be obtained of Miller Bros. whose 
address is given elsewhere in this number of the NATURALIST. 
MEASUREMENT OF ANGULAR ApeRTURE.— For those students 
whose stands have not a graduated rotating base, Prof. T. D. a 
coe recommends a plan especially applicable to the larger cia 
achromatic condensers which cannot be as conveniently @ per 
for the method advised by Dr. Carpenter. He places the me 
nation horizontally, with a gas flame several feet distant behi 
to furnish nearly parallel light, holds a card across the Cen" ‘id 
the front lens so as to bisect the cone of light, and seat r 
pencil marks the edges of the illuminated portion of the "a 
common protractor is used to measure the angle of the pe 
drawn. Though chiefly useful to measure the eye-plec? ty used 
achromatic condensers (Webster’s condenser, ete.) aS aoe „otimes : 
with diaphragm, ete., in position, the method may be see ‘ 
convenient for objectives of low power. Prof. Bian pi reli- T 
can be applied to objectives as high as 4 in. of 110 a - 
able within one or two degrees. i, 
