1890.] 



On the Heat of the Moon and Stars. 



485 



that the column must be as slender as possible so long as its stability 

 is not impaired. The further the focus is outside the telescope, the 

 larger must be the plane mirror. As this is a disadvantage both on 

 account of expense and of obstruction of useful rays, I have ma- 

 terially reduced the size of the flat by placing it well on one side of 

 the axis. The resulting want of definition I found by calculation to 

 be for my purpose of no importance. So long as all the rays fall on 

 the sensitive surface, which in some cases is 1 mm. square, the defini- 

 tion is sufficiently good. With the very low power eyepiece that 

 I use I cannot, detect anything more than a point in the image of a 

 star. 



The radio-micrometer which I made for this purpose is unusually 

 large and massive, and, as the suspended circuit is hung in a narrow 

 hole drilled in this great mass of solid metal, differences of tempera- 

 ture in different parts of the circuit can hardly be produced by out- 

 side influence, as, for instance, the observer's body. Any heat so 

 applied must first unequally warm the massive cast-iron box ; it 

 must then be imparted to the solid metal radio-micrometer (nearly 

 80 lbs.), which is only supported by five points forming a geometrical 

 slide, and then it will only be the difference of temperature in the 

 solid metal between points not much more than a quarter of an inch 

 apart, which by imparting some portion of itself to the suspended 

 circuit will cause any indication of heat or cold. If, however, 

 radiation reaches the sensitive surface through the small hole drilled 

 horizontally in the solid metal, and this can only come from the 

 limited field of view of the telescope, then one portion of the circuit 

 will be independently warmed, and a corresponding deflection will 

 be produced. 



The only part of the instrument which reaches outside the cast- 

 iron box is a slender tube which carries the cork and pin from which 

 the circuit is suspended by a very fine quartz fibre. This tube is 

 made of glass ground' into the metal of the radio-micrometer. The 

 object of using glass is to , prevent any loss of heat from the ap- 

 paratus into the outer air. This part is also boxed in by an easily 

 removable double hox of wood. In making this tube I blew a hole 

 in the side and 'thickened it with a welt of glass about half an inch 

 above the level of the box lid. To the face of the welt previously 

 ground flat is cemented a piece of a plano-convex spectacle lens, 

 which forms an image of its own on the scale and at the same time 

 brings the light reflected from the plane mirror behind it to a focus 

 on the same scale. The mirror, which consists of a piece of the 

 thinnest microscope cover-glass silvered at the back, produces with 

 this arrangement an image so good that tenths of millimetre can be 

 easily read. To prevent the delicate circuit from being influenced 

 by draughts in the telescope, a tube containing diaphragms is fastened 



