3< a IN STARRY REALMS. 



of heat, for the greater part of the rays of heat are of 

 the invisible type ; though no doubt some of them are also 

 risible, as the red portions of the spectrum. I must also 

 add that within the last few months wondrous possibilities 

 have been opened up as to the discovery of innumerable 

 other rays of much greater length, which do not directly 

 appeal to any senses that we have been provided with. 

 But with such extraordinary rays as those which can pass 

 through a stone wall, and be refracted by a prism of 

 pitch, we have not at this moment to do ; though they 

 are of the most intense interest, and possibly will admit 

 of remarkable astronomical applications. The rays with 

 which photography is concerned are mainly or largely 

 of the invisible type, but they are rays of high refran- 

 gibility : they lie out beyond the violet. 



Thus it happens that the rays from the star which are 

 competent to excite an impression on the plate are partly 

 in the visual portion, but chiefly in the invisible part 

 of the total radiation. Now we can see another reason 

 why the photograph may, and indeed must, largely extend 

 our conceptions of the extent of the universe. It will 

 grasp and depict light which would be utterly wasted so 

 far as vision is concerned, for even were these rays poured 

 in torrents into our eyes they could excite no sense of 

 vision ; and consequently all stars whose radiation did 

 not contain a sufficient admixture of visual rays, no matter 

 how copiously they diffused these ultra-violet rays, would, 

 to the eye, be invisible in the most powerful telescope, 

 though capable of being recorded by a photograph. It 

 will thus be manifest that the grounds which the new 

 method furnishes of increased powers to the astronomer 



