306 JRoyal Astronomical Societj/. 



the following. If the object-glass with the micrometer attached be 

 tilted, the place of the image of the star upon the micrometer will 

 be disturbed by a small quantity, unless the plane of the micrometer 

 be at one certain distance from the object-glass. It is therefore an 

 important matter to determine what that certain distance will be. 

 It is easily seen that if the plane of the micrometer pass through one 

 of the points called focal centres, this condition is satisfied. For a 

 ray from a vertical star passing through the focal centre (the object- 

 glass being inclined), will be refracted in a parallel vertical direction 

 to the quicksilver, and will then be reflected back from the quick- 

 silver in the same line, and will by refraction be made to pass again 

 through the same focal centre ; and, supposing the distance of the 

 quicksilver to be properly adjusted, so that the image of the star is 

 formed on the micrometer, that image will be at the focal centre 

 whatever be the inclination of the object-glass. The place of the 

 focal centre may be determined by an apparatus, in which the object- 

 glass is planted in a frame that admits of being slid in a direction 

 perpendicular to its plane, the sliding- cell being upon aboard which 

 turns in its own plane on a pin ; a beam of light is directed upon 

 the lens through two narrow slits ; and a telescope is placed on the 

 opposite side of the object-glass to receive the light ; when, by trial 

 of sliding the frame, a position is so determined that, upon rotating 

 the turning-board, through a large angle, the position of the beam 

 of light as seen in the telescope does not change, it is then certain 

 that the focal centre is in the axis of rotation. If the micrometer 

 can be conveniently fixed at this distance from the object-glass, the 

 accidental inclination of the object-glass will be unimportant ; if the 

 micrometer is at any other distance, there will be a very small cor- 

 rection to the measures, depending on the inclination of the object- 

 glass ; and it will be proper that a small spirit-level be attached to 

 the object-glass frame for the measure of the inclination. Perhaps 

 in any case this addition will be prudent. 



The adjustment to focal length will depend upon nothing but the 

 distance between the object-glass and the quicksilver ; and a power 

 of altering this distance must be retained. It is proposed to do this 

 by moving the tube, in which the object-glass turns, up or down 

 by a rack-and-pinion motion, the tube and its load being as nearly 

 as possible balanced by a lever-counterpoise. It is also proposed 

 that a smaller quicksilver-trough, communicating with the larger, 

 should carry a float, from which a light stalk should rise by the side 

 of the object-glass frame ; if this stalk be made of the same material 

 as the micrometer, a scale upon the stalk will indicate the value of 

 the micrometer-scale as corrected for thermal expansion, and as 

 affected by any change of focal length. 



Although such an instrument may be adapted to the observation 

 of any stars which pass within a field of view expressed by the 

 breadth of the object-glass, yet some conveniences of fixation are 

 gained by limiting it to the one star y Draconis. The following are 

 the details of mounting proposed by the Astronomer Royal. The 

 micrometer necessarily requires two metallic bars crossing the object- 



