636 AMERICAN ASSOCIATION ADVANCEMENT OF SCIENCE. 
expense, giving sufficiently good black-ground and oblique-light effects 
for small microscopes. This lens is used for transparent illumination of 
both fields of the binocular with 1-2 or 1-4-inch objectives (the Webster 
condenser, with its smallest centre stop, and graduating diaphragm, at- 
tains the same end in a much finer manner), but much of the light passed 
even by it is detrimental, and its performance may be improved by a cap 
of card or paper, pe: over it, having a horizontal opening, or a vertical 
stop (Plate 5, figs. 2 and 3), one of the openings in Fig. 3 being closed 
when oblique light is Pianist A horizontal opening of —ÓÀ width 
may be easily combined with the brass mounting of the spotted le 
1 usin objective or similar tna: as achromatic s 
the horizontal slit is still more applicable. It (Plate 5, fig. 4) may be 
added, for instance, to the stop-plate of Powell and Leland's achromatic 
condenser, or -— in the supplementary aperture of Ross' 4-10 con- 
denser, or in small microscopes screwed in between the lenses of a con- 
densing Suite. pe ae t stops must be used for different angles of 
o9 
regulated by the Paci plate, or by Zentmayer's graduating dia- 
phragm, or Brown's iris diaphragm which. instead Z the diaphragm- 
plate, should be combined with condensers of this clas 
ut the real value of this stop, and the real ease ba banding the light 
in the every-day work of the d microscope, is attained with 
the large-lens condensers, with which a 1-4 of 75?, or, when more resolv- 
ing power and less depth of field is d ed, a 4-10 of 110° to 120°, can be 
as easily managed as a 1-inch, both fields being softly and evenly lighted. 
Plate he fig. 6, ind placed in the same fr vias or the stop-plate may 
b modified as to furnish a horizontal slit as in Plate 5, fig. 7, the 
Nap of the slit hens controlled by the aaa spi An ad- 
justable slit may be extemporized by using a nen edge of card in 
width and curved direction of the slit any serious inconvenience in prac- 
prin 
e graduating di e We ia for facility of use and certainty of results, 
teg fairly superseded the original wheel of apertures; perhaps the time 
nothing left to remind us of our circular diaphragm-plate. If the optic- 
ians would give us something having the general arrangement of the 
EAE AS ETON ENE EEE NEN E aa sau E E N S OE, 
E Eg REENE A Pu Nou eS FEAN REET, E 
