On a Series of Lantern Slides. By Dr. A. Clifford Mercer. 315 
axial tube to the centre of the camera front. In photomicrographing 
an opaque object, a diaphragm cap is slipped over the front of the 
objective. Thus a diaphragm comes to he supported about half-way 
between the object and the objective : it there cuts off all rays from 
the object or stage outside the area to be photographed, and is analo- 
gous to the hood of a portrait or landscape lens. When an objective 
has a short working distance, a piece of dead black card or paper with 
a central hole, instead of the cap, covers all the object except the area 
to be photographed. In photomicrography, as in all critical optical 
work, it is important here and everywhere to shut out or suppress, so 
far as possible, every non-effective or wandering ray of light. The 
wainscoting of my room is black ; the woodwork of the apparatus is 
black ; the walls and floor are non-actinic ; by day I always photo- 
micrograph with covered windows ; and at night I have no light in 
the room but that on the revolving board. 
In slide 52 (fig. 36) is also seen a small cubical box projecting 
from the front board of the camera. Within the box is a mirror 
which reflects the light from a white card, substituted for the ground- 
glass, through a single opera -glass fastened to the nearer side of the 
box. With this addition to the apparatus in place, I can sit at the 
side of the table, arrange the object and illumination with the eye at 
the side tube of the Microscope, withdraw the mirror from the axial 
tube and then, without getting up, look through the opera-glass and 
roughly focus an image on the white card. Focusing is then com- 
pleted at the other end of the camera in the usual way by means of 
the fine-adjustment rod and pulley. At the left are seen two screw- 
clamps which are used in fixing the camera back at any required 
distance from the Microscope, the exact distance being shown by a 
scale cut on one side of the camera bed. The eye end of the tube is 
seen to be supported by an adjustable standard, a necessity in delicate 
work with high powers. 
Slide 59 (fig. 38) shows an arrangement of apparatus for photo- 
micrography with transmitted axial sunlight. To the left is an upright 
supporting a horizontal axial tube with a portion of its upper half cut 
away to allow an alum or water cell, an ammonio-sulphate of copper 
cell, a disc of ground glass, and diaphragms, one or more of these, to 
stand in the lower half. The Microscope is seen to the right. Between 
is an 8-in. portrait lens so supported by an upright that its distance 
from the Microscope can be varied at pleasure, by sliding the upright 
along the revolving board. The lens is attached to the upright by 
two sliding boards, one sliding vertically and one horizontally, for the 
purpose of centering. The portrait lens was originally intended to 
eliminate diffraction phenomena in accordance with the Woodward 
method ; * but the lens is also occasionaly used instead of a substage 
condenser. Outside a south window, slide 57 (fig. 39), is a Stratton 
Blouthly Microscopical Journal, vi. (1871) p. 170. 
