Further Notes, dec. By E. M. Nelson. 
3 
With regard to the management of the Microscope, the adjustments of 
the substage condenser for centering, &c., are precisely similar to those 
when a lamp is used in the ordinary way. In this case the slit in 
the screen D takes the place of the lamp-flame. When the condenser 
has been centered and the slit in the screen D brought to the centre 
of the field, the whole of the Microscope (or mirror if one is used) 
will probably require some movement before any light appears in the 
slit in the screen D. Even after the light has been obtained it will 
not he sufficient that the image of the lens R appears in the slit as in 
fig. 5, hut it should be accurately centered by moving the whole 
Microscope until it is seen as in fig. 6. 
I have placed another lens Q equal in focus to R on the other 
side of the prism P, which enables one with a more compact apparatus 
to obtain a spectrum the width of which equals the length of the 
slit, because the slits in the screens S and D are placed at the prin- 
cipal foci of the lenses, instead of double that distance as before. 
This plan also secures a more powerful illumination which enables 
work to be carried on deeper down into the blue. It is important 
that the screen D should be placed accurately in the focus of the lens 
R. It is a good plan to test the quality of the light by holding a 
white card between C and D before adjusting the Microscope. The 
size of the pencil after emergence from the screen D should be noted, 
and care should be exercised to see that the rays fill the back lens 
C of the substage condenser. 
I have found that there is a considerable gain if the instrument 
is used direct, in other words the mirror abolished and the axis of the 
Microscope placed centrally in the pencil emerging from the screen 
D. To set this up is more troublesome, but by returning to my 
former wooden model it becomes easy. The whole apparatus, includ- 
ing the Microscope, stands on a plank of wood or base-board, but the 
lamp L, the bull’s-eye B, the screen S, the lens Q, the prism P, and 
the lens R, are attached to another board which is pivoted on the base- 
board, and which is made capable of a slight rotation about the point 
P. The screen S and the second lens Q are fixed, and the prism P, 
which is on a turn-table, has a clamping screw so that it also 
may be fixed when the position for minimum deviation has been 
determined. 
The lens R is as before capable of rotation about P, and it is 
likewise fitted with a clamping screw. The screen D is, however, 
fixed to the base-board on which the Microscope stands. The reason 
for this is that instead of moving the lens R in order to cause the 
movement of the spectrum across the slit in the screen D, which tends 
rather to disturb the adjustments, the whole board is rotated about its 
pivot P, and the light of the required colour brought in a line with 
the slit D. The whole apparatus is capable of being inclined to suit 
the inclination of the Microscope. 
