482 



Prof. C. V. Boys. 



[Apr. 24, 



finally, that Dr. Huggins has kindly lent me a silver-on- glass mirror of 

 nearly 16 inches aperture and very short focus (67*8 inches) of his 

 own construction, which was exactly what I wanted for this purpose. 

 I accordingly prepared a design for a mounting of this mirror, such 

 that, no matter in what direction the telescope might be pointed, the 

 focus of the rays from a star should be always at one fixed point at 

 which the receiving surface of the radio-micrometer could be sus- 

 pended. I should say here that in this one particular the radio- 

 micrometer is at a disadvantage by comparison with a thermopile or 

 bolometer ; it must not be tilted, it cannot be fixed into an eyepiece 

 and pointed and moved about at will ; it must be level, though it 

 may, with its lamp and scale, be carried on a level platform and 

 turned about a vertical axis. This disadvantage, which, however, 

 disappears when the whole apparatus is designed for and is made to 

 suit the radio-micrometer, is more than counterbalanced m by the 

 absence of connecting wires with a thermo-electric junction at every 

 binding screw and by the absence of the galvanometer, which is ever 

 ready to give indications on its own account. 



In the design of the mounting I have, to a large extent, been 

 obliged to follow certain lines. Thus it is evident, if a large sidero- 

 stat is not to be had so that the telescope maybe fixed, which would be 

 the most convenient plan to adopt, that the mounting must be 

 altazimuth, that the horizontal and vertical axes of motion must in- 

 tersect, and that the focus, which must be just outside the tube, must 

 be on or close to the vertical axis. With this arrangement trunnions 

 to the telescope would be very inconvenient, as they would necessi- 

 tate a long and awkward curved arm, which would prevent the 

 telescope from being turned over from east to west, or from north to 

 south. Accordingly, I have adopl ed the plan suggested by Mr. W. 

 H. Massey, of carrying the telescope by a large disc on its side, rest- 

 ing and turning in an under-cut groove. Pig. 1 shows the chief 

 details of construction finally determined upon, and fig. 2 the finished 

 instrument in position. The radio-micrometer must be protected 

 from stray radiation, and from the effects of hot and cold air currents. 

 I therefore arranged that the space at the back of the under-cut disc 

 should be made in the form of a box with a movable lid, all of thick 

 cast-iron, so that the changes of temperature inside should be 

 sufficiently gradual ; then the nature of the radio-micrometer would 

 prevent such changes from producing any disturbing effect. The 

 floor of this box continued away from the telescope forms a con- 

 venient base for holding the lamp (ether oxygen lime light) and 

 scale. 



The telescope tube carried by the disc on its side must be balanced 

 about the centre cf this disc. The large cast-iron weights carried by 

 four rods are in such a position as to balance it exactly ; moreover the 



