33 REPORT— 1873. 



angular position. When the desired position is approximated and the trigger 

 relieved, the pin drops into the nearest hole,- and the endless screw is then used 

 for final setting. 



The diagram explains the various matters of illumination. 



From a lamp hanging upon the end of the declination axis is sent a beam of 

 slightly divergent light through this axis, which is hollow ; this slightly divergent 

 beam is utilized for six different purposes, three portions of it being reflected out 

 in different directions to illuminate portions of the declination circle, of which one 

 is for a long reader for setting from eye end, and the other two for micrometer 

 microscopes subdividing the 10' division of circle into single 1" arc. 



None of these are shown in diagram ; but the other three purposes for which the 

 light is utilized, viz. for position-circle, bright field illumination, and dark field 

 illumination of micrometer, are shown. 



The position-circle illumination is very simple (see fig. 1) ; a single reflector R, 

 attached to the inside of the tube, directs a constant beam of light on the back of the 

 glass circle at P. 



The bright field illumination is effected by a very small central reflector R', 

 which sends the light directly into the field of the micrometer. 



This method is, the author believes, now generally considered to give the best 

 results, and has, as far as he is aware, but one disadvantage, viz. that the arm 

 which supports the small mirror produces a little diffraction, and consequently 

 deterioration of definition. 



This objection is in some measure reduced by making the arm and mirror 

 removable at pleasure by pulHng or releasing a string, so that while actually 

 observing, it can be removed and replaced instantaneously. 



In devising the dark field illumination, the author started on the hypothesis that 

 there were two essential points to keep in view, viz. that the lines should be illumi- 

 nated 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 eyepiece should not be injured. 



The best results were obtained ,by placing four prisms of total reflection roimd 

 the field of the micrometer, just behind the wires, and of such an angle that the 

 light thrown upon them shoidd be reflected upon the wires at an angle such as 

 is shown in diagram fig. 2, where W is the position of wires in focus of objective. 



In order that this scheme of illumination should be carried out etiectually 

 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 eft'ected in the following way : 

 a portion of the slightly divergent beam of light, shown in fig. 2, proceediug from 

 the lamp on the declination axis is passed through a very low-power convex 

 lens, I, which renders the beam slightly convergent. 



This is not necessary, but a mere matter of convenience, as it reduces the neces- 

 sary size of the reflector and lens afterwards required. The light is now taken up 

 by a reflector, E, 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 eyepiece-tube.'=, i ; 

 hence it is passed through a piece of glass of a peculiar shape, P P, which is called, 

 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 fiunular prism lens is twofold : — 



1st. It has to alter the direction of the beam of light, before diagonally thrown 

 across the tube R X, to that parallel to the axis of the telescope ; aud 



2nd. It is necessary that it should have a slightly converging efiect 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, p, p, and illu- 

 minates 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 eyepiece-tube is sufficient to completely destroy 

 the efiect of this illumination. The difficulty, however, has been completely 

 removed in this way. 



