702 



NATURE 



[February 25, 19 15 



rather darker. The "L" near the middle is deep 

 blue, the word "mark" is white, and the g^round 

 they are on is yellow. The book covers are red. 

 It will be noticed how the panchromatic plate g^ives 

 the yellows better than the orthochromatic plate, 



(a) Ordinaiy Piai 



{!>) Isochromatic Plate and Yellow (X4) screen. 



feCAXETTEER, 





ATLAS 



(<;) Panchromatic Plate and Yellow (K 3) Screen. 

 Fi(-.. 2. 



and gives the reds, which the orthochromatic plate 

 renders in the "Gazetteer " and "Atlas " as if the 

 red paper was of the same colour as the black 

 printing- ink. Other effects will be clear without 

 further description. 



NO. 2365, VOL. 94I 



Panchromatic plates have been made commer- 

 cially for fifteen years or more, but the plates of 

 the last few years are very superior to their pre- 

 decessors. Some makers issue more than one kind,, 

 the variation so far as colour sensitiveness is 

 concerned being chiefly as to the extent of the 

 red sensitiveness. Some are sensitive only tO' 

 about the red lithium line or the C line in the 

 solar spectrum. Others are sensitive to beyond 

 the A line or the red potassium lines, and plates 

 can be supplied that are sensitive considerably 

 further into the infra-red. For the correct repre- 

 sentation of coloured objects infra-red, and everv 

 the extreme red, sensitiveness is only harmful, 

 just as ultra-violet sensitiveness is, because these 

 radiations are dark to the eye. It is well to choose 

 a plate for this purpose that is not sensitised to 

 the extreme red, and it is necessary to use a colour 

 filter that cuts off the ultra-violet (some of the 

 older yellow filters did not do this) and reduces 

 the intensity of the blue so that it does not produce 

 an undue effect. With a modern panchromatic 

 plate a suitable filter will require the exposure 

 to be increased perhaps only five times, as com- 

 pared with the exposure necessary for a white 

 object without the filter. This does not mean 

 that such a plate and filter will give a sensitive- 

 ness curve that will exactly coincide with the 

 luminosity curve of the solar spectrum, but it is 

 a very near approach to it, so near that for 

 general purposes it is more important to see 

 that the present degree of perfection is main- 

 tained, rather than that it Is increased, though 

 of course we hope for still better results in due 

 time. 



There are many uses to which colour sensitised 

 plates and colour filters may be put besides the 

 correct representation of colour luminosities in the 

 photography of objects, spectrum photography, 

 and In connection with three-colour reproduction 

 processes. Given black and white and any single 

 colour, it is possible to photograph the three so 

 that the colour Is represented as if it were 

 either black or white. Take the simple 

 case of a drawing or manuscript in black 

 ink on white paper with alterations in red ink. 

 The red alterations can be made to appear as 

 black as the black ink by using blue (or blue and 

 green) light only, that is, a light which has no 

 red in it. An ordinary plate, which is sensitive 

 only to blue, will give this result. But the red 

 alterations can be made to appear as white, andj 

 therefore to disappear, by using red light only, 

 that is, a pLate sensitised to red in conjunctionj 

 with a red colour filter. Engineers' drawings inj 

 blue, as when copied by the ferroprusslate process,, 

 give poor contrast on ordinary plates because the 

 active light Is chiefly blue, but with a red sen- 

 sitised plate and red (or yellow) filter the blue 

 becomes practically bl.ack. Similarly it is often 

 possible in photography to eliminate stains on 

 documents, or to find an inscription that has faded 

 Into Invisibility. In this latter case it may be well 

 to use extreme measures, as Including as much 

 ultra-violet as possible In one experiment, and the 



