February i8, 19 15] 



NATURE 



677 



chromatic " or " isochromatic " photography, or 

 whatever it may be called, cannot yet even be 

 regarded as an absolute matter; but where the 

 discrepancy in the use of " ordinary " plates is of 

 the order of a thousand to one, there is plenty of 

 room and need for improvement, before getting-, 

 as it were, within sight of perfection. 



"When the spectrum is photographed on an 

 ordinary plate, the green and red, which are bright 

 to the eye, produce little or no effect ; they might 

 as well be black, while the blue and ultra-violet, 

 which are dark and black to the eye respectivelv, 

 produce a considerable effect, as if they were 

 bright. Similar results are obtained with ordinary 

 objects ; slate roofs, being bluish, come much too 

 light; bricks, being red or reddish, come much 

 too dark; grass and green foliage too dark, and 

 so on. The plate is sensitive to all these colours, 

 but it is very much too sensitive to blue, or not 

 sensitive enough to green and red. By causing 

 the light that falls upon the plate to pass through 

 a colour filter that will reduce the brightness of the 

 blue light to about one-thousandth part of its 

 intensity, and increasing the exposure proportion- 



Ordinary Plate. 



I so- Plate without screen. 

 Fig. I. 



ately, the green and red will be given an opf)or- 

 tunity to act, and the result will be much im- 

 proved. To increase exposures to one thousand 

 times the usual length may sometimes be possible 

 (say two minutes instead of the tenth of a second), 

 but the undesirability of such an increase need not 

 be pointed out. 



Dr. H. W. Vogel, in 1873, discovered that by 

 the application of certain colouring matters, it 

 was possible greatly to increase the sensitiveness 

 of plates to green and red light. About ten years 

 later the application of this principle began to be 

 made a commercial matter, and Messrs. Edwards 

 and Co. secured the patent rights in this country. 

 These isochromatic or orthochromatic plates were 

 a great step in advance. The three illustrations 

 (Fig. i) were prepared many years ago from a 

 design for which the writer was indebted to Mr. 

 B. J. Edwards. The cross and the disc in the 

 original are of such dark shades of blue, that from 

 a distance of from one to three yards, according to 

 the sensitiveness to blue of the eyes of the person 

 looking at the card, they appear quite black, while 

 the ground colour is a bright yellow. On the 

 ordinary plate the yellow comes out darker than 



NO. 2364, VOL. 94] 



the almost black blue. The improvements ob- 

 tained by using an orthochromatic plate, and then 

 by the use of a colour filter to reduce the blue 

 light in proportion to the yellow, are shown in 

 the reproduction. 



There are two or three matters in connection 

 with the use of such means as these to get 

 variously coloured objects represented according- 

 to their luminosities that may be pointed out as 

 well from this example as from any other, bearing 

 in mind that they represent general principles. 

 Such plates as these ("ortho-" or " iso-chromatic ") 

 are often, if not generally, stated to be sensitive 

 to yellow. This is misleading. Spectrum yellow, 

 as already stated, is negligible in these matters. 

 All objects that are yellow are yellow because thev 

 absorb blue, and send red and green light to the 

 eye. Yellow light is a mixture of red and green. 

 These plates have their sensitiveness increased to 

 green and not to red. If, therefore, we so arrange 

 our colour filter as to get full correction for 

 yellow, that is, that yellow and blue shall be 

 correctly represented according to their luminosi- 

 ties, we throw the correction that ought to be 

 borne by the green and red jointly 

 entirely on to the green, and this 

 coloiir is therefore over-corrected. 

 «^^^ .- >-.t.Ai Greens will therefore be represented 

 ^^^^^ too light. On the other hand, the 



^pH^I increased sensitiveness does not ex- 



^^^B^^ tend over the whole of the green, 



it is chiefly in the yellowish^ireen. 

 and the curve of sensitiveness shows 

 an important depression in the re- 

 gion that may be roughly indicated 

 as being between E and F. Pure 

 yellowish-greens tend, therefore, to 

 be over-corrected on this account 

 also, but what is perhaps of more 

 importance is that a green that 

 comes in this depression of sensitiveness will be 

 under-corrected and come out too dark. This is 

 not a mere theoretical difficulty, for M. Callier, 

 who is a most careful investigator, finds that the 

 green of pine trees largely corresponds to this de- 

 ficient sensitiveness, while that of grass corre- 

 sponds rather to the specially sensitised yellowish- 

 green. Therefore these two greens are repre- 

 sented as more different in brightness than they 

 really are. 



These facts illustrate the difficulties that result 

 from the fact that specially sensitised plates have 

 not an evenly graded sensitiveness. There is the 

 maximum for the plate, and a new maximum for 

 the new comf>ound introduced. Such irregxilarity 

 might be compensated by a complex colour filter, 

 but of course only approximately and with much 

 trouble and considerable increase of the necessary 

 exp)osure. 



The "ortho-" or " i so-chromatic " plates of 

 commerce are generally of the type just discussed, 

 and are sensitised by erythrosin or a similar sub- 

 stance. In a second article we shall refer to 

 " panchromatic " plates and other matters. 



Chapman Joxes. 



Iso- Plate with screen. 



