Dr. C. K. Akin on Calcescence. 39 



an observer placed at the end of the tube, which being parallel 

 to the mirror reached just a little beyond it, might see the inner 

 surface of the platinum-foil without casting a shadow on the 

 mirror, although the mirror faced the sun in the most advanta- 

 geous position^ viz. so as to concentrate the solar rays at the prin- 

 cipal focus. 



Next after the mirror and the tube, the most important piece 

 of apparatus consisted in the diaphragm or diaphragms by which 

 the solar rays were to be sifted. To avoid every objection, it 

 was intended to have these diaphragms a little larger than the 

 mirror itself, and to connect each as it might be employed late- 

 rally and all round with the mirror, so as to allow of the inci- 

 dence of no rays upon the platinum except such as had previously 

 passed through the diaphragm. Diaphragms of this size, how- 

 ever, and of the qualities required, were not to be procured ; and 

 it was thus necessary to employ much smaller ones, which were 

 placed between the mirror and the platinum in frames supported 

 by stands. In this manner some of the rays incident had to pass 

 twice through the diaphragm, first on incidence, and next after 

 reflexion. To obviate the objection arising from the incidence 

 of side-light upon the platinum, it was proposed to employ a 

 truncated tin cone, the base of which was formed by the dia- 

 phragm itself, whilst the apex gave admittance to the tube 

 supporting the platinum*. In this manner, no rays could 

 reach the platinum but such as had previously passed through 

 the diaphragm ; but, on the other hand, the cone itself would 

 have cast a shadow on the mirror, if the latter was placed in its 

 most favourable position. 



The diaphragms to be employed were of three kinds — (]) red 

 glass, (2) black glass, and (3) iodine dissolved in bisulphide of 

 carbon contained between thin sheets of glass. As regards red 

 glass, we succeeded at last in procuring from Birmingham panes 

 of glass about 1' square, which, tested by the pocket-spectro- 

 scope, cut off all rays of greater refrangibility than the line D. 

 When combined with green glass of the deepest tint, red glass 

 of this description made the sun appear red ; which showed 

 that the green glass was slightly transparent to red rays, although 

 when tested by the pocket-spectroscope it cut off all rays of less 

 refrangibility than the line D. Hence it is evident that the 

 analysis by the spectroscope in the case of the red glass also 



which the piece of looking-glass was mounted. When the looking-glass was 

 properly adjusted, the hole was stopped up light-tight by means of a brass 

 cap, which fitted on the cylindrical borders of the hole. 



* The cone was pierced in two places, to admit of the wires of the bat- 

 tery, properly insulated from the substance of the cone. 



