Dr. Woods on the probable Nature of the Sun's Body. 25 



posure. The examination of these pictures showed that they 

 were of different sizes, the sraallest being produced by the shortest 

 exposure ; and that they increased in extent with the length of 

 time the aperture was open up to a certain size. The centre of 

 the picture was apparently intensely acted on_, as it had the 

 appearance of being what photographers called "burnt/' And 

 this deep spot was surrounded by a ring of light not so darkly 

 marked. The " burnt'^ centre increased in size, not in depth of 

 intensity, with increased length of exposure ; the ring about it 

 also increased, but not in proportion to the enlargement of the 

 centre. A piece of red glass placed before the aperture of the 

 camera renders the sun's action less powerful, and allows the 

 pictures to be taken less rapidly. These experiments are a fur- 

 ther proof that the light from the centre of the sun acts more 

 energetically than that from the edges ; the latter requiring a 

 longer time to produce as much effect as the former on a photo- 

 graphic surface. 



I thought that these experiments might furnish a ground for 

 obtaining evidence of the probable nature of the sun's envelope. 

 I determined to try whether ^ame would affect a sensitive plate 

 after a manner similar to the sun ; and if so, whether a solid 

 body producing light would differ in its action. I therefore 

 exposed a prepared surface in a camera in the focus of a lighted 

 candle, and also of a gas-jet. In both cases the action was ex- 

 actly similar to that of the sun, but more marked, as to variety 

 of extent, in the size of the pictures produced, because the light 

 was not so powerful, thereby allowing more leisure in the mani- 

 pulation. The picture of the flames in one second made a slight 

 impression, in two seconds the impression increased in size and 

 apparent depth of action, and so on, up to 15 seconds, when the 

 picture produced was about three times the size of that taken in 

 one second in the case of the gas-jet. In numerous experiments 

 I have made with flames, caused to burn steadily lest the waver- 

 ing might influence the result, I have always found that their 

 action on the plate was similar to that of the rays from the sun's 

 disc, viz. an increased extent of picture for an increased period 

 of exposure. 



It now remained to try what effect a solid body giving out 

 light — not reflecting it — would produce. It was not easy to 

 find a means of heating a solid body sufficiently high to get a 

 light capable of acting on a sensitive plate. I tried iron heated 

 to whiteness, and platina in the flame of a gas-jet, but neither 

 affected the plate in a camera. The lime-light, however, acted 

 well. A piece of lime acted on by the oxy-hydrogen blowpipe 

 was rendered luminous, and a picture of it thrown on the pre- 

 pared surface by the camera. In one second a deeply-m?irked 



