38 I)r. C. K. Akin on Calcescence. 



mirror, the wooden tube referred to, the diameter of which 

 was about 2", was closed by means of two thin plates of slate 

 cut out at the centre, and between which the platinum-foil in- 

 tended to be rendered incandescent was placed. These plates of 

 slate were designed to close up the tube and render it light-tight, 

 holding at the same time the platinum in its place, and prevent- 

 ing as much as possible caloric conduction*; they rested on a 

 slight circle or shoulder on the interior of the tube, and were 

 prevented from slipping or dropping by means of an ivory ring 

 screwed on the exterior of the tube. This ivory ring bore, at 

 two points diametrically opposite, two angular pieces of brass of 



this form | , the shorter and vertical limbs of which had 



binding screws attached, to make connexion with wires leading to 

 a galvanic battery, whilst the longer and horizontal limbs, near 

 their free extremities, bore each a vertical screw with a flat base 

 made of platinum. The bases of these screws could be screwed 

 down, and made to rest upon the exposed face of the platinum- 

 foil — the upper slate being cut out nearly elliptically, whilst the 

 lower slate was cut out circularly, the diameter of the circle 

 being about equal to the short axis of the ellipse. Thus, when 

 the ivory ring was connected with a galvanic battery, the current 

 passed from one of the angular pieces already described, through 

 the platinum-foil (of about 1" breadth, and 1 to 1^" length), to 

 the other angular piece f. 



The tube thus partially described was about 1' long, and 

 could be connected with another and similar tube of rather 

 greater length than the semidiameter of the mirror — the two 

 tubes, when connected, forming an angle of 90°. At the interior 

 of this angle a piece of looking-glass was placed, making angles 

 of 45° with each of the two axes of the tube J. By this means, 



* To some extent the plates of course conduct away the heat of the pla- 

 tinum. This circumstance, however, is not without advantage, at least in 

 the case of such experiments as before mentioned, in which it was desired 

 to apply solar radiation subsequent to galvanic heating. In these experi- 

 ments it is desirable that the platinum should remain hot, as long as pos- 

 sible, after the galvanic heating has ceased — a result which was materially 

 aided by the contact of the platinum with a substance such as slate, which, 

 having been itself heated at first by the galvanic current, afterwards re- 

 tained its heat much longer than the platinum, and partially conveyed it 

 back to the platinum. 



t The thicknesses of the foils varied from ¥ Jo" to -goVo" > some of them 

 were of silver platinized. The screws which rested on the foils were suffi- 

 ciently apart not to interfere with the cone of rays incident on the foils. 



X The whole of the tube consisted of three parts — one angular, and the 

 two others straight. By joining the latter two parts together, of which one 

 supported the foil and the other the ocular diaphragm, the tube could be 

 used for direct vision, without the intervention of reflexion. The angular 

 or bent piece was sheathed on the outside in brass, and the angle itself was 

 cut off by a circular section, leaving a hole into which was fitted a cork, on 



