November 28, 1895] 



NATURE 



91 



observed by Dr. Halm, at Blackford Hill, as follows: — 

 November i8, i8h. 26m. 14s. G.M.T. ; K.A. I3h. 48m. 15s., 

 J)ecl. 0° 48' i8""8 N. The movement of the comet is in a 

 direction south-east by east, and it now rises about 4 a.m. Dr. 

 Lamp states that the comet is moderately bright, but not visible 

 to the naked eye ; it is round, with a central condensation, and 

 a straight tail ^Ast. Nach. 3318). 



Variable Star Clusters. — Harvard College Observatory 

 Circuiar, No. 2, announces that an extraordinary numlier of 

 variable stars has been discovered in certain globular clusters 

 which have been photographed by Prof. Bailey at Arequipa 

 with the 13-inch Boyden telescope. At least eighty-seven of the 

 stars in the cluster M3 (N.G.C. 5272), in Canes \ enatici, have 

 been found to be variable, and in some cases tlie change of light 

 mounts to two magnitudes or more. In the cluster M5 

 X.Ci.C. 5904), forty-six variables were found, out of 750 stars 

 \amined, so that they form about six per cent, of the whole ; 

 nf the sixteen stars, contained in a circle no" in diameter, six 

 are variable. Smaller numbers of variables have been found in 

 other clusters, but in other cases not a single variable has 

 l)cen detected out of the hundreds of stars which have been 

 photographed ; the conditions of the search, however, not 

 taking account of long period changes. In general, no varia- 

 liles have been found within about one minute from the 

 I cntres of the clusters, on account of the closeness of the stars, 

 and none are more than ten minutes distant from the centres. 

 Some of the newly-discovered variables have short periods, in 

 some cases of only a few hours. Thus, five photographs of 

 N.Cl.C. 5904, taken at intervals of an hour on July i, 1895, give 

 for the magnitude of a star about three minutes of arc preceding 

 the centre of the cluster, 14-3, I3'5, 13*8, I3'9, and I4"3 ; four 

 plates, taken at similar intervals on August 9, gave the 

 magnitudes 14 "2, 14 "6, 14 '8, and i5'o. 



ON A METHOD OF PHOTOGRAPHS IN 

 NATURAL COLOURS} 



TN 1861 Clerk Maxwell described a method of colour photo- 

 graphy, based upon his experiments on the theory of colour 

 vision, and made the following experiment. Three photographs 

 of a coloured object were taken through three several coloured 

 solutions giving images which separately represented the object 

 as it would be seen by each of the three sets of colour nerves 

 jxistulated by Voung. When these were superposed the 

 original colours of the object were reproduced, save for the defect 

 that the red and green components suffered from the insensitive- 

 ness of the photographic plate of Maxwell's time to the longer 

 wave-lengths. Maxwell added the remark that when the photo- 

 graphic plate was improved as regards sensitiveness to the less 

 refrangible rays, the representation of colour would be 

 improved. - 



Since Maxwell's day the colour blindness of the plate has been 

 almost completely remedied, thanks to the discovery of Vogel, 

 and it is now possible, proceeding on the lines laid down by 

 Maxwell, to produce by triple projection upon the screen a 

 picture which may be illusively like nature. J-'or the application 

 of modern resources and the suggestion of photographing to the 

 colour vision curves by special colour screens, we have to thank 

 Mr. Ives. 



Composite colour photography deals with the subjective 

 reproduction of all visible wave-lengths in two stages ; a photo- 

 graphic analysis and an optical synthesis. In the first opera- 

 tion the several wave-lengths are caused to produce three 

 separate photographic images according to their physiological 

 activity in exciting the supposed fundamental red, green, and 

 violet sensations. That is, if the image bears, for example, a yellow 

 colour (suppose such a yellow as the spectral yellow near the D 

 line), one of the plates must record an image of the object 

 having a density of silver deposit corresponding to the degree in 

 which this wave-length can excite the red-seeing nerve, and a 

 second must acquire a density corresponding to the degree in 

 which this same wave-length can excite the green-seeing nerve. 

 The third plate records no impression, for the wave-lengths near 

 I) excite no violet sensation; but this yellow sensation is the 



• Abstract of .1 paper read before the Royal Dublin Society, by Dr. J. 

 Joly, F.R.S. 

 - "On the Theory of Three Primary Colours.' " Collecteil P.ipers," p. 449. 



NO. I361, VOL. 53] 



resultant of two physiological eflfects only, a red and a green 

 sensation in certain proportions obtained by colour measurements 

 effected upon normal colour sight. We have now obtained three 

 negatives possessing densities of silver deposit corresponding to- 

 the degrees in which the three several fundamental colour sens- 

 ations are stimulated. These degrees of density will be inter- 

 preted as degrees of transparency in the positives. The first 

 positive, if backed with a red gla.ss, will transmit a quantity of 

 red light corresix)nding to the intensity of the physiological 

 excitation of redness in the " red " nerves ; the second, backed 

 with green, similarly represents the stimulation of the " green " 

 nerves by the yellow colour of the object ; the third positive is 

 backed with blue-violet glass, Imt is quite opaque, and no violet 

 light is transmitted through it. The projection now of all three 

 images superposed upon the screen forms the second stage of 

 the procedure ; the optical synthesis of the original colours. 

 The eye regarding the superposed image receives, in fact, the 

 same amounts of red and green sensation, and exjieriences the 

 same absence of violet sensation which would have attended the 

 formation of the image of the original object upon the retina. 



This process, if accurate reproduction of colour is sought, 

 necessitates the use of two distinct sets of colour selective 

 screens ; for the analysing screens will by no means possess the 

 tints ultimately required in the optical synthesis. This is 

 evident since the measurements on colour vision reveal that the 

 wave-lengths near D are more strongly stimulative of red 

 sensation than are the purely red exciting wave-lengths near C, 

 and the wave-lengths again diminish in their power of producing 

 stimulation of the " red " nerves on the more refrangible side of D. 

 Hence, in order to photograph the wave- lengths of the spectrum, 

 we require to produce a greater photographic effect by the D 

 wave-lengths than by the C wave-lengths, and a photographic 



CB 



effect diminishing above D in the same degree as the power of 

 the waves to excite the fundamental red sensation diminishes. 

 To effect this analysis of the light a screen transmitting as pre- 

 dominant wave-length, a wave-length near D must be used for 

 obtaining the image which is to represent the appreciation of 

 light peculiar to the "red" nerves. Such a screen has a 

 yellow-orange colour, which is not the sensation excited in or 

 transmitted by the " red " nerves. In the optical synthesis this 

 must afterwards be represented by a C red colour. The same 

 remarks apply to the other .screens. 



Maxwell's curves (Fig. i) are not colour sensation curves 

 (Abney : "Colour Vision," Tyndall Lectures, 1895), and it is 

 misleading to speak of the foregoing method as effected on colour 

 sensation curves. Maxwell's curves represent, in fact, the subjec- 

 tive synthesis of the spectrum out of three chosen wave-lengths— a 

 red, a green, and a blue- violet. The question as to how far one or 

 all these chosen wave-lengths may excite more than the one set 

 of nerves remains over, and indeed can only be gone into by 

 examination of abnormal colour vision. In Koenig's curves of 

 colour vision, colour sensations are plotted. These are shown 

 in the named curves of Fig. 2. 



If, from the knowledge afforded by Koenig's curves of the 

 compound nature of the green sensation. Maxwell's curves be 

 examined with reference to their suitability to serve the purposes 

 of the photographic method, it will be found that, assuming 

 Maxwell's E green to excite the proportionate amounts of red 

 and violet sensation revealed by Kcenig's curves, a correct 

 synthesis of the F green by Maxwell's curves is impossible. 

 Although such a comparison is not strictly allowable owing tOi 



