486 



Prof. C. V. Boys. 



[Apr. 24, 



to it and projects nearly as far as the side of the telescope tube. 

 The diaphragms are of such a size as to limit the view from the 

 sensitive disc to the cone of rays. A tube of this kind was used by 

 Langley to protect his bolometer from the influence of draughts. 

 It is nothing more than the old toy through which you can drop a 

 pencil but not blow out a candle. 



The arrangements of lamp and scale are hardly worth describing 

 in detail. I will merely say that I used the ether light to avoid the 

 necessity of having two kinds of compressed gases, which, in the 

 country, would otherwise be necessary. The ether light is exceed- 

 ingly convenient for this purpose, but I can hardly recommend it, 

 for, though I used the safety burner supplied by the makers, the first 

 thing it did, owing to my inadvertently turning off the oxygen, was 

 to explode with a loud report, and the copper box, after striking the 

 roof of the house, fell close to me, not burst, it is true, but blown 

 into a more or less bulbous form. Moreover, on frosty nights, the 

 ether box is so cold that the gas- which comes out requires no more 

 oxygen, so that an explosive gas is being led from the reservoir 

 direct to the burner. Under these circumstances any stoppage of the 

 oxygen supply would at once cause a violent explosion. As it is, it is 

 impossible, on a cold night, to . stop the gas without its exploding 

 down to the tap which turns it off. 



I provided, between the ether box and the burner, a pair of extra 

 regulating taps, which, by a touch, will turn off' the oxygen and turn 

 down the (so-called) hydrogen, or turn them up again for an observa- 

 tion. By this means waste of ether and oxygen is avoided, and the 

 limes last a long time. 



I have arranged a slow motion in azimuth, which is more con- 

 venient when observing on or near the meridian, but none in altitude, 

 as that would have been troublesome to make. The motion in alti- 

 tude is rather stiff, but being used to it now I can follow a star in 

 any part of the sky, step by step, without difficulty. 



As I have already said, the tube was made by Messrs. Davy, 

 Paxman, and Co., of Colchester; the stand and heavy fittings were 

 made by Messrs. Thomas Horn and Sons, of Gray Street, Waterloo 

 Road. S.E., engineers, and with regard to this part of the work, I 

 must express my great satisfaction at the way the work has been 

 carried out. Nothing is done for show, but every working surface is 

 true, and works freely without shake. I believe this is the first thing 

 of the kind that Mr. Horn has made ; if he had done no other class 

 of work but this he could not have done it better. The radio-micro- 

 meter and all the odd fittings and adjustments I made myself, and these 

 parts have given no trouble. Into the details of the mounting of the 

 mirror and certain minor adjustments it is not worth while to enter 

 at length. It is sufficient to say that every part is capable of 



