Book Xotice. 



47 



To test the faithfulness with which the plates had recorded 

 the colours, the yellow screen was removed, and the colours on 

 the plate compared with those on the ground glass of the camera 

 — their identity was striking. This was tried because it had 

 been pre\douslv observed that the screen shortened the spectrum 

 of ordinary daylight more than that of the arc-light, which 

 extends considerably further at the more refrangible end. The 

 only difference which this seems to make is that it shortens the 

 exposure appreciably. 



While these photographs were being taken, an interesting 

 experiment was tried by Mr. J. Archyll Jones, B.Sc. the Prin- 

 cipal at the Technical College. An ordinary lantern plate was 

 exposed and developed as the colour-plates are, except that 

 ruby hght was used in the dark room ; the image was then 

 treated with the acid permanganate, washed, developed for a 

 second time, then fixed with ' Hypo ' solution such as is gener- 

 ally employed for fixing plates. The result was a fine positive, 

 obtained direct. 



To further test the colour process, a selenite plate was 

 placed in front of the objective. This, of course, changed 

 the tints, giving at times most beautiful results ; but the plates 

 recorded even these exquisite colours, and a fine photograph of 

 Luxulyanite was obtained under these conditions. When 

 working with the higher powers, as when interference-figures 

 are being photographed, some httle care has to be taken with 

 the alignment of the beam of light and the axis of the micro 

 tube : but a crystal of Aragonite cut at right angles to the 

 acute bisectrix, and placed so that its opticaxial plane made an 

 angle of 45 degrees with the principal sections of the Xicols, 

 which were crossed, readily yielded its famihar figure in its true 

 colours. Any descriptions of these plates, without illustrations 

 in colour, is indeed ' Hamlet,' without the ' Prince of Den- 

 mark ' : so I forbear. 



I have not yet tried a photograph of the spectrum, but it 

 does not seem likely that the plates would fail even there. 



(iuide to the Specimens of the Horse Family [Equidce) exhibited in 

 the Department of Zoology, British Museum (Natural History). London, 

 1907. 42 pp., price i/- 



. The Trustees of the British Museum are certainly earning the gratitude 

 of air naturalists by the way in which they are placing work of this character 

 in the hands of the public at so small a cost. ^Ir. R. Lydekker has written 

 the present Guide. It is a fascinating narrative, full of facts of value to 

 the student of evolution, and is illustrated by twenty-six figures. 



igaS February i. 



