visible in the Solar Spectrum. 3 



of the solar photosphere, must be considered as deductions from 

 such portions of this research as have hitherto been completed. 



I had already, in my former paper, remarked that the Fraun- 

 hofer-lines in the solar spectrum were, so to speak, an inversion 

 of the bright lines in the electrical spectrum, and that an ex- 

 planation of the lines in the one system would in all probability 

 also furnish an explanation of those of the other, according to 

 the principle laid down in that dissertation. I endeavoured to 

 apply the often-named principle to the bright lines in the elec- 

 trical aerial spectrum, and to the Fraunhofer-lines in the solar 

 spectrum, though I had not then, nor have I since, found any 

 more accurate correspondence between the two. I endeavoured 

 to explain to myself the causes of this circumstance, principally 

 by the differences of density and the different states of elasticity 

 of the air during the passage of the solar light through the 

 atmosphere, and at the moment of its being heated by the 

 electric spark. This difference, however, cannot exist in the 

 case of the gases which envelope the sun, and the presence of 

 which is evidenced by the corona and the so-called protuberances 

 visible during total eclipses of the sun. 



It may now generally be taken for granted that the Fraun- 

 hofer-lines for the most part originate in the sun's photosphere* 



* It has been urged as an objection to the hypothesis that the Fraun- 

 hofer-lines belong principally to the sun, that they ought in that case to 

 appear stronger and more distinct when the rays come from the edge, than 

 when they come from the centre of the disk, which, according to Forbes's 

 observation, was not the case. 



Forbes's observation took place under the different stages of a solar 

 eclipse, under which circumstances I conceive it would be very difficult to 

 preserve the appearance of the spectrum accurately in the memory. I 

 therefore considered that experiment to be worth repeating. 



I used for this purpose an optical' theodolite with two telescopes, one of 

 which was furnished with a slit-opening to admit the sunlight. The height of 

 this orifice was reduced to about 3 millims., and the sun's image was pro- 

 jected upon it from a Dollond object-glass of 3 '02 metres focal distance. 

 The diameter of the image thus formed was 28 millims. ; and by allowing the 

 rays from different parts of this image to fall successively upon the opening, 

 it was easy to see whether the Fraunhofer-lines underwent any change. 

 Any very remarkable change I could not discover. All that I fancied I 

 could remark was, that the intensity of the spectrum-light is somewhat 

 less when the ray comes from the edge, than when from the centre of the 

 disk ; and this is evidenced by the circumstance that the fainter Fraun- 

 hofer-lines show themselves in the latter case comparatively stronger, 

 whereas, when the light comes from the centre of the solar disk, the fainter 

 lines will sometimes even totally disappear, while the stronger lines, as for 

 example, some of the iron-lines, appear with correspondingly increased 

 brilliancy. As we know from Kirchhoff's experiments that an increased dif- 

 ference of intensity between the source of light and the absorbing gas is 

 favourable to the distinctness of the lines in the gas-spectrum, it would 

 seem that this observation, if confirmed, is not repugnant to what we 

 already know concerning the absorbing-power of gases. 



B2 



