Royal Astronomical Society. 305 



north of the zenith of Greenwich. The Astronomer Royal therefore 

 has been compelled to endeavour to devise an instrument which shall 

 be firm in its connexions, and shall also be api^licable to the obser- 

 vations of stars on both sides of the zenith. The following is the 

 construction which he proposes for this purpose. 



III. The proposed reflex zenith telescope is founded upon these 

 considerations. If an object-glass be placed with its axis vertical ; 

 and if a pencil of light fall on it from a star near the zenith, and 

 pass from the object-glass with its axis still inclined to the vertical, 

 but with the rays of the pencil in a state of convergence ; and if a 

 trough of quicksilver be placed below it at a distance somewhat less 

 than half the focal length of the object-glass ; the pencil of light 

 will then be reflected from the quicksilver with its axis still inclined 

 in the same degree to the vertical, and with the rays still in the same 

 state of convergence, and will again pass through the object-glass, 

 and will form an image of the star at a very short distance above the 

 object-glass, and at a distance from the axis of the object-glass de- 

 pending on nothing but the star's zenith distance and the focal 

 length of the object-glass. Although we cannot fix on the axis of 

 the object-glass, yet we know that if the object-glass is turned 

 through 180°, the image will now be formed at an equal distance 

 from the axis of the object-glass, but in an opposite direction rela- 

 tively to the frame of the object-glass ; and therefore the distance 

 between the two positions of the image, as measured by a micrometer 

 attached to the frame of the object-glass, will be double the distance 

 of either image from the axis of the object-glass, and will therefore 

 be a measure of the star's zenith distance. The peculiar advantage 

 of this construction is, that it requires no firmness of connexion ex- 

 cept that of the micrometer with the frame of the object-glass. The 

 mercury-trough may be totally unconnected with the rest of the 

 instrument. The firmness of support of the object-glass is unim- 

 portant ; for, however much the object-glass is pushed sideways 

 (giving the same movement to the image of the star), the micrometer 

 is equally pushed sideways, and the measure of the image is not 

 disturbed. 'J'he peculiar disadvantage is, that the light must be 

 reflected from quicksilver, and must pass again through the object- 

 glass, and must be transmitted through a four-glass eyepiece with 

 a diagonal reflector, so that the whole loss of light will be consider- 

 able. But this disadvantage appears insignificant in comparison 

 with the advantage. 



Theoretically, the place of the image will be aff^ected bj'^ a local 

 fault in that part of the object-glass through which the rays pass the 

 second time. But, practically, the existence of such a fault is un- 

 likely ; and its effect, if any existed, would be proportional to the 

 distance of the image from the object-glass, and therefore small : and 

 moreover it could be ascertained and measured by previous experi- 

 ment on the object-glass. No injury, therefore, to the results, and 

 no real inconvenience to the observations, would arise from such a 

 fault. 



The only risk to which this construction appears to be exposed is 

 Phil. Mag. S. 3, Vol. 35. No. 236. Oct. 1849. X 



