Transparent only to Ultra-Violet, Liglit. 263 



white objects and objects not white. I have also photo- 

 graphed a collection of rocks and minerals with ultra-violet 

 light and with yellow light. In the negative taken by yellow 

 light there is a great deal of contrast and detail, especially 

 in the marbles and conglomerates, while in the negative taken 

 by nltra-violet light all this is absent, the white specimen? 

 coming out very black, with everything else of a thin and 

 almost uniform grey. 1 hope in the near future to have an 

 opportunity of making some lunar photographs on a large 

 scale, the only instrument at my disposal at the present time 

 being the nine-inch equatorial of the University. Photo- 

 graphing by ultra-violet light appears to diminish the con- 

 trast between all objects not white, and to increase the 

 contrast between white objects and those not white. I do 

 not wish to be hasty in drawing conclusions, but it appears 

 to me to be probable that the more luminous portions of the 

 lunar surface, if not as white as plaster-of-paris, must at least 

 be much whiter than grey sandstone. In Plate IV. fig. 4 are 

 reproduced two photographs of the same landscape, taken at 

 the same time and under similar conditions of illumination, 

 the one (A) taken on an orthochromatic plate by yellow light 

 through a screen of dense aurantia, the other (B) taken on 

 an ordinary plate by ultra-violet light. 



The absence of contrast between the chimneys and walls in 

 B is especially noticeable in the right-hand part of the picture. 

 I tried various times of exposure, and the picture reproduced 

 is the best of the lot. Another curious effect is the almost 

 complete absence of shadows in the ultra-violet picture (it 

 was taken in full sunlight like the other), showing that most 

 of the ultra-violet light comes from the sky, which is what 

 we should expect, though we should hardly anticipate that 

 the effect would be so pronounced. This is best seen on the 

 monument and on the snow in the middle distance. The 

 increase of " atmosphere " in the ultra-violet picture is very 

 marked. It is so strong that under-exposed plates fog in the 

 shadows of objects not over one hundred yards from the 

 camera, a circumstance which shows the great scattering 

 power of the air for these short waves. The two pictures 

 are also interesting as showing that our eyes have developed 

 a maximum sensibility for that region of the spectrum which 

 shows terrestrial objects in strongest contrast. Nitroso- 

 dimethyl-aniline is the only substance, other than the ordinary 

 aniline dyes, that I have examined thus far, and 1 feel very 

 hopeful of finding, among the large number of allied sub- 

 stances, absorbing media even more transparent to the ultra- 

 violet radiations than the one which L have described in this 

 paper. 



