1914-15.] Reflective Power of Pigments in the Ultraviolet. 147 
also appeared black, and zinc carbonate greyish. Schweinfurth green 
(cupric aceto-arsenite) and copper carbonate were equally darker than in 
visible light, and dark and black inorganic salts had in general the same 
appearance as in visible light. Of the organic salts the alkaloids showed 
many inexplicable differences in behaviour towards these rays. Ninety 
flowers were photographed, and all except the yellow ones appeared 
uniformly black. The yellow flowers could be divided into two classes : 
those in one class followed the general law and absorbed these rays strongly,, 
those in the other class almost completely reflected them. Michaud and 
Tristan term the latter “ultraviolet flowers,” and suggest that they will 
form four or five per cent, of a random choice of flowers of all colours. 
For the present work a Thorn ton-Pickard Imperial camera was used,, 
fitted by R. and J. Beck, Ltd., with a quartz lens system of two components, 
which is similar in shape and arrangement to a rapid rectilinear lens. Each 
component has a focal length of 30 cms. and an aperture of 2*2 cms. The 
camera is of the triple-extension type, with a total extension of 38 cms.^ 
and carries quarter-plates. A maximum magnification of 1'6 can be 
obtained with the double lens, or 0’3 if only one component is used. 
The method naturally occurring to me for obtaining monochromatic 
pictures in the ultraviolet was to use colour filters ; these in general allow 
of reasonably short exposures in the visible spectrum. However, with the 
exception of the silver film already referred to, there are as yet no mono- 
chromatic filters known suitable for the ultraviolet. A few preliminary 
experiments were made with a silver film deposited on the surface of a 
quartz plate. It tarnished rapidly and had some small holes which trans- 
mitted white light. It is possible that by combining special sources of 
illumination with suitable absorbing solutions a series of monochromatic 
pictures may be obtained, but after some preliminary work it was resolved 
to leave the investigation of this matter to a future research. 
The next method tried was to place the objects under investigation in a 
darkened room and illuminate them with ultraviolet light from a mono- 
chromatic illuminator; the constant deviation nickel-mirror spectroscope 
already described in these Proceedings * was used. The source of light was 
an iron arc placed on a ledge outside the window and suitably enclosed to 
prevent extinction by wind. The light was admitted to the slit of the 
spectroscope through a hole in a shutter. The plate carrier of the spectro- 
scope was replaced by an adjustable slit, by means of which a beam of light 
of any desired width could be transmitted. The wave-length of the trans- 
mitted light was varied by rotation of the focussing mirror, and could be 
* R. A. Hoiistoim, Proc. Roy. Soc. Edin., xxxii, p. 40 (1912), 
