THE EOYAL ARTILLERY INSTITUTION. 
355 
“ This objection I have in some measure reduced by making the arm and mirror 
removeable at pleasure by pulling or releasing a string, so that while actually 
observing, it can be removed and replaced instantaneously. 
“ In devising the dark field illumination, I started on the hypothesis that there 
were two essential points to keep in view—viz., that the lines should be illuminated 
on both sides (not one), and that the angle at which the light should be thrown 
upon the wires should be very great, so that the blackness of the field as seen 
through the eye-piece should not be injured. 
“ I found that the best result is obtained by placing four prisms of total reflection 
round the field of the micrometer, just behind the wires, and of such an angle that 
the light thrown upon them should be reflected upon the wires at an angle such as 
is shown in the diagram Tig. 2, where W is the position of the wires in the focus 
of the objective. 
“ In order that this scheme of illumination should be carried out effectually from 
the light of a single lamp hanging on the declination axis, it is necessary that a 
certain annular portion of the micrometer which embraces these prisms should be 
constantly illuminated from this lamp, and this is effected in the following way:— 
A portion of the slightly divergent beam of light, shown in Fig. 2, proceeding from 
the lamp on the declination axis, is passed through a very low power convex lens, l t 
which renders the beam slightly convergent. 
“This is not necessary, but a mere matter of convenience, as it reduces the 
necessary size of the reflector and lens afterwards required. The light is now 
taken up by a reflector, R, within the tube, and directed towards the eye-end at 
such an angle that it crosses the axis of the telescope just at the inner end of the 
eye-piece tubes, X; hence it is passed through a piece of glass of a peculiar shape, 
PP, which I call, for want of a better name, an annular prism lens. This piece of 
glass has a hole cut in it large enough to admit the whole pencil of light from the 
object glass. 
“ The use of this annular prism lens is twofold 
“ 1st. It has to alter the direction of the beam of light before diagonally thrown 
across the tube, AX, to that parallel to the axis of the telescope ; and 
“2nd. It is necessary that it should have a slightly converging effect to reduce 
the size of the illuminated circle it produces. 
“This arrangement, so far, performs perfectly in all but one particular* It 
throws a strong beam of light constantly upon the four prisms, pp, and illuminates 
the lines well; but although no direct light can enter into the field from the mirror 
placed so far out of the cone of rays from the objective, still the light thrown 
against the side of the eye-piece tube is sufficient to completely destroy the effect 
of this illumination. The difficulty, however, has been completely removed in 
this way - 
“ I should first mention that the eye-piece or micrometer tube is made double—■ 
an outer parallel tube and an inner taper one—and it is between these two that it 
is required that the light should be brought to the four prisms or micrometers, 
any light shining into the inner tube doing mischief by injuring the blackness of 
the field. 
“ On the lens used to give a slight convergence to the light is placed a circular 
opaque disc, 0, of a certain size easily ascertained. A lens, 7, of a suitable focus 
being then placed near the reflector, an image is formed of that opaque disc just 
over the eye-piece tube at X, and of such a size, when properly adjusted, that no 
light can possibly enter the inner tube. 
“ Thus, while not a single ray of light can by any possibility enter the inner 
tube, a flood of light is sent down between the inner and outer tubes, and directed 
upon the four prisms in whatever angular position they may be. 
“ It only remains to say that both the intensity and colour of the light for both 
