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XVII. Preliminary Notice of a new Sunsliine-Recorder. 

 By Herbert M'Leod*. 



BEING somewhat doubtful of the accuracy of the pub- 

 lished reports of the duration of sunshine, in the 

 summer of 1880 I tried to devise some apparatus by which 

 the light, instead of the heat of the sun, would be used to 

 produce the record of sunshine. Several trials were made; 

 and in some of them clockwork was employed to move strips 

 of sensitive paper. The simplest form of apparatus I have 

 recently tried again, using the ferroprussiate paper now much 

 employed by engineers for copying tracings, and which is 

 preferable to silver paper, first, in consequence of its 

 cheapness, and, secondly, because of the ease with which the 

 image is fixed, as for this purpose washing in water is 

 sufficient. 



The apparatus consists of a camera so fixed that its axis 

 is parallel to the polar axis of the earth, the lens pointing 

 northwards. Opposite the lens a silvered sphere is placed. 

 The rays from the sun are reflected from the sphere through 

 the lens of the camera on to the sensitive paper, on which a 

 distorted image of the sun is formed : and the positions of the 

 lens and sphere are so arranged that the image is a linear one 

 and radial. By the motion of the earth the image is carried 

 round in a circular arc, tracing a curve on the sensitive 

 paper. In the instrument at present constructed, the sphere 

 is a chemical round-bottomed flask silvered inside, and about 

 95 millim. in diameter ; the distance from flask to lens is 

 76 millim., and from lens to sensitive paper 152 millim. ; the 

 lens has a focal length of 90 millim. and an effective 

 aperture of 22 millim. 



The circle traced by the sun in June is about 120 millim. 

 in diameter. An impression is made on the paper by exposure 

 for only 10 seconds ; and when the lens is covered for one 

 minute, a light line is produced in the circular band, so that 

 the paper is sufficiently sensitive in the present apparatus to 

 register short gleams of sunshine, and also the passage of 

 small clouds. When the sun is shining through light clouds, 

 an impression is produced on the paper, but somewhat 

 blurred, and of a much less intense blue colour than is 

 obtained by bright sunshine. The time-scale is made by 

 drawing from the centre of the circular band radial lines, 

 containing between them angles of 15°, each of which 

 represents one hour of time. 



* Communicated by the Physical Society. Read June 28, 1884. 



