Moon Colours. 389 



They are, on fine nights, more like the effect of white light seen 

 through delicate transparent screens of different hues, and 

 could probably be better imitated by transparencies than by 

 drawings on paper, like the star colours of Admiral Smyth, 

 which ought by the way to be imitated in transparent glass. 



Mr. Birt has shown us a series of tints on paper which, 

 could only be imitated by careful hand-colouring at considerable 

 expense, and he also allowed us to examine an instrument 

 which he terms a " homo-chromoscope," which he brought 

 before the Astronomical Society in 1861, and which is well 

 worth serious consideration. It consists of a number of tints 

 in circular patches painted on a glass slide, and moving in a 

 frame, so that amy one can be brought to a central aperture, 

 through which it is illuminated from behind, and observed in 

 front. The light is intended to be reflected by a sheet of white 

 paper suitably placed to catch the rays from a small lamp, and 

 the observer would look with one eye through his telescope, 

 and with the other at the iC homo-chromoscope" until he found 

 a tint corresponding with that under his notice upon the moon's 

 surface. The instrument thus devised by Mr. Birt is only in an 

 experimental stage of its existence, but we are anxious to call 

 attention to it, as it could only be brought to a satisfactory 

 state by the co-operation of other observers, and by securing 

 for it, when completed, a sufficient sale. 



The first thing to do would be to get a moderate number 

 of good observers, whose eyes are tolerably free from any form 

 of colour-blindness, to agree upon the principal tints. From 

 experiments we have made with a fine refractor of 3 inches, 

 and with a 6^ inch silvered mirror, we find that ordinary 

 observers differ in the amount of yellow they see in the moon, 

 in the quantity of purple they notice, and in their estimation 

 of the greenish tint, which is often invisible to ordinary 

 eyes, and which probably does not always exist. Dull, but 

 dear ochrey-yellows, neutral tint blues, sometimes passing 

 into browns, at others into purples and purple greys, all 

 more or less differing from terrestrial colours, and therefore 

 difficult to describe in terms usually applied to them, are what, 

 perhaps, would be generally agreed to exist ; but it would be 

 worth while for a " Moon Committee" to request a couple of 

 good colour artists to paint on glass, in transparent tints as near 

 as possible, the tints of Mare Serenitatis and Tranquilitatis, and 

 send copies to ten or twenty observers, and ask for their 

 reports. Many persons find themselves much assisted in esti- 

 mating star colours or brown tints, by being told how others 

 see them, and this must not be regarded as exciting a pre- 

 judice in favour of the tint thus selected, but rather as indi- 

 cating what delicate peculiarity is to be looked for. If a few 



