100 A NEW DARK-FIELD MICROMETER. 



revolve in a suitable frame, so that the luminous points may 

 bebi'oughtinto parallelism with thepair of stars to be measured, 

 and the angle read off from a graduated circle on the rim^ 

 the zero point being first ascertained by revolving the scale 

 until a star shall run along the single line of plate B. The 

 difference of readings will give the position angle with 

 reference to the meridian, it being supposed that the telescope 

 is mounted equatorially. 



The foregoing will, I think, make the pi'inciple clear. 

 Dimensions will depend very much on the size of the telescope. 

 In my case, the glass slides are 7in. x 4in., the opening of 

 the circle or ring 4in. The telescope is a Newtonian reflector 

 — speculum 8|in. The apparatus is fixed perpendicularly on 

 the telescope tube at a distance (towards the speculum end) 

 of 19|in. from the eye-tube, this distance being adopted for 

 convenience, as giving a value of ^ sec. of arc with the 

 power I generally use for double stars. The sliding glass, 

 slip fits into a brass sliding frame, or carrier, which moves by 

 a rack and pinion. A scale of lOO divisions is engraved on 

 the side of the frame, answering to the length of the glass, 

 slide. (See A and P, Pig. 1.) 



For the glass slides I prepare a coated slip three lengths in 

 one, ruling the diverging lines the whole length of the slip,, 

 from the angle at one end to an opening of about 3|in. at the 

 other. This slip is then cut into 3 lengths (commencing from 

 the point of the angle), each length being exactly equal to 

 the 100 divisions on the frame. This gives scale readings to 

 100, 200, 300, the glasses being interchangeable in the frame. 

 The whole arrangement, with its graduated circle, revolves in 

 the frame which supports it, by a pinion in the support, 

 "woi-king in a toothed wheel on the circle. My apparatus is 

 fitted with a small electric lamp (2| candle), with a contact 

 conveniently near the eye-piece. At the back of the lamp is a 

 concave reflector, to throw a parallel beam of light upon the 

 scale. It is of advantage to frost the back surface of the 

 glass (next to the lamp). The coated surfaces should be next 

 each other without rubbing. 



The measurement is effected, not by direct coincidence, but 

 by conpm-ison. Supposing we are working without a driving-. 

 clock, the " ghost," as I will call it {i.e., the image of the 

 points), is brought to about the middle of the field, and the 

 star brought into position with it. The circle is then revolved 

 until the " ghost " is sensibly parallel with the line joining 

 the components of the star, and the slide moved to correspond 

 with the distance. When these adjustments are perfect, as 

 the star approaches and recedes from the " ghost," the four 

 points will form a perfect parallelogram. (Fig. 2.) 



Practically it will be found that the eye is very sensitive to 



