NATURE 
308 [May 23, 1912 
upon a fine line screen, of which the opaque lines | in the way of the construction of the necessary 
are three times as wide as the clear interspaces, and 
forming an image on this screen by means of a lens 
with a prism just in front of it. The prism spreads 
each white line into a complete spectrum, and is so 
calculated that the spectra lie next each other on the 
focussing screen without interspace. If instead of 
white light falling upon the line screen we allow 
coloured light to fall upon it, only those spectrum 
colours of which the line in question is composed | 
appear on the focussing screen, the colours which 
are wholly or partially missing from the spectrum of 
Fic. r.—Micro-spectra camera on stand. 
(Made to instructions by Alfred B. Allen, 
20, Endell Street, London, W.C.) 
white light being represented by spaces wholly or 
partially dark. 
In taking the photograph, the image of the coloured 
object is projected by means of any ordinary objective 
lens on to the line screen, the image of which is in 
turn projected by the second lens with the prism in 
front of it on to the photographic plate placed in the 
position of the focussing screen (Fig. 2 shows diagram- 
matically the general optical arrangement). The plate 
must be approximately equally sensitive to all colours, 
so that the resulting negative is completely darkened 
when acted upon by any colour in its full intensity, 
and partially darkened where the incident colour is 
weakened. A lantern slide positive from this negative 
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Fic. 2.—General optical arrangement shown diagrammatically. 
will, of course, show the reverse effect, being com- 
pletely transparent where the colour has acted with 
full intensity, of partial transparency where the colour 
has acted less strongly, and opaque where the colours 
were missing, 7.e. in those parts coincident in position 
with the spectrum colours of white light that were 
not present in the object photographed. When there- 
fore this positive is placed in the exact position of 
the negative, and white light is projected through the 
apparatus, it acts as the desired mask to block out 
those colours that are not wanted, and the picture is 
reproduced in the original colours. 
Like so many other scientific problems, however, 
whilst the theory was simple, in practice difficulties 
NO. 2221, VOL. 89] 
| 
| successively overcome. 
apparatus (Figs. 3 and 4) arose at every turn, and 
matters were further complicated by the necessity of 
keeping the camera within portable limits. To indi- 
cate one of the main sources of difficulty, an ordinary 
glass prism produces a spectrum widely extended in 
the violet and blue region and crowded up at the 
yellow and red end, an effect very detrimental to the 
proper rendering of the latter colours. This was over- 
come by the use of a compound prism specially com- 
puted to give a spectrum in which the colours are 
evenly distributed, as in a grating spectrum. The 
introduction, however, of a thick prism of this kind 
introduced aberrations of all kinds, both in the images 
| of the object and of the spectra, which had to be 
Sectional plan of micro-spectra camera viewed from above 
(optical system). 
Movement Lateral 
rotating the micrometer 
optical system. movement. 
Fic. 3.—Section of micro-spectra camera. 
It was, for example, found 
necessary to place the line screen (which has 372 lines 
per inch) at a slant to bring the spectra all over the 
field sharply into focus, a cylindrical lens is used in 
front of the prism to correct for astigmatism, the 
front of the camera is placed at the proper angle to 
prevent wedge distortion, a narrow prism behind the 
first objective brings the object sharply into focus, 
and so on. The objectives used in the camera are 
two 75 mm. Zeiss micro-planars. A field lens is 
interposed between the first objective and the line 
screen to direct the light towards the second objective. 
The whole optical system can be slightly rotated by 
means of a milled head on the left-hand side of the 
| camera in front; at the back is another milled head 
securing slight lateral movement, and a lever above 
the viewing screen (not shown in Fig. 1), permits of 
Fic. 4.—Section of optical system. 
A, Zeiss 75 mm., micro-planar objective on focussing 
mount; B, spectacle prism; C, field lens ; 
1), line screen or grating in adjustable frame ; 
E, 75 mm. micro-planar objective ; F, compound 
prism ; G, cylinder spectacle lens, 120” focus. 
a slight backward. or forward movement of mm. 
These three movements are necessary to enable the 
lantern plate to be brought to the exact position of the 
negative, but correct registration is easily secured in 
a few seconds—the readings can, moreover, be noted 
on the positive. 
Besides the method of viewing the picture on the 
focussing screen of the camera, which requires a 
strong artificial light source, the pictures may also 
be viewed direct on the line screen by means of a 
magnifying eyepiece, for which purpose ordinary day- 
light or a weak. illuminant suffices. This method in 
practice does not, however, yield quite such good 
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