486 KEPORX— 1889. 



don't care to mention. This is the theory •wliicli I have always advocated, viz. that 

 the dj-e by its reduction acts as a juicleus on which a deposit of silver can take 

 place. It met with opposition ; a rival theory which makes the dye an ' optical 

 sensitiser' — an expression which is capable of a meaning which I conceive contrary 

 to phj'sical laws — being run agaiust it. The objection to what I may call the 

 nucleus theory is less vigorous than it has been, and its diminution is due 

 perhaps to the more perfect understanding of the meaning of each other by those 

 engaged in the controversy. To my mind, the action of light on fugitive dyes 

 is one of the most interesting in the whole realm of photography, as eventually 

 it must teach us something as to the structure of molecules, and add to the 

 methods by which their coarseness may be ascertained. Be the theory what it may, 

 however, a definite result has been attained, and it is now possible to obtain a fair 

 representation of the luminosity of colours by means of dyed films. At present the 

 employment of coloured screens in front of the lens, or on the lens itself, is almost 

 an essential in the method when daylight is employed, but not till some dye is 

 discovered which shall make a him equally sensitive for the same luminosity to 

 the whole visible spectrum will it be possible to make orthochromatic photography 

 as perfect as it can be made. The very fact that no photograph of even a black 

 and white gradation will render the latter correctly must of necessity render any 

 process imperfect, and hence in the above sentence I have used the expression 

 ' as perfect as it can be made.' 



The delineation of the spectrum is one of the chief scientific applications to 

 which photograph}^ has been put. From very early days the violet and ultra- 

 violet end of the spectrum have been favourite objects for the photographic plate. 

 To secure the yellow and red of the spectrum was, however, till of late years, a 

 matter of apparently insurmountable difficulty ; whilst a knowledge of that part 

 of the spectrum which lies below the red was only to be gained by its heating 

 effect. The introduction of the gelatine process enabled the green portion of the 

 spectrum to impress itself on the sensitive surface ; whilst the addition of various 

 dyes, as before mentioned, allowed the yellow, the orange, and a portion of tlie red 

 rays to become photographic rays. Some eight years ago it was my own good 

 fortune to make the dark infra-red rays impress themselves on a plate. This last 

 has been too much a specialty of my own, although full explanations have been 

 given of the methods employed. By preparing a bromide of silver salt in a 

 peculiar manner one is able so to modify the molecular arrangement of the 

 atoms that they answer to the swings of those waves which give rise to these 

 radiations. By employing this salt of silver in a film of collodion or gelatine 

 the invisible part of the spectrum can be photographed and the images of 

 bodies which are heated to less than red heat may be caused to impress them- 

 selves upon the sensitive plate. The greatest wave-length of the spectrum to 

 which this salt is sensitive so far is 22,000 X, or five times the length of the visible 

 spectrum. The exposure for such a wave-length is very prolonged, but down to a 

 wave-length of 12,000 it is comparatively short, though not so short as that 

 required for the blue rays to impress themselves on a collodion plate. The colour 

 of the sensitive salt is a green blue by transmitted light ; it has yet to be 

 determined whether this colour is all due to the coarseness of the particles or to 

 the absorption by the molecules. The fact that a film can be prepared which by 

 transmitted light is yellow, and which maybe indicative of colour due to fine particles, 

 together with an absorption of the red and orange, points to the green colour 

 being probably due to absorption by the molecules. We have thus in photography a 

 means of recording phenomena in the spectrum from the ultra-violet to a very 

 large wave-length in the infra-red — a power which physicists may some day turn 

 to account. It would, for instance, be a research worth pursuing to photograph the 

 heavens on a plate prepared with such a salt, and search for stars which are 

 nearly dead or newly born, for in both cases the temperature at which they are 

 may be such as to render them below red heat, and therefore invisible to the eye 

 in the telescope. It would be a supplementary work to that being carried out by 

 the brothers Henri, Common, Roberts, Gill, and others, who are busy securing 

 photographic charts of the heavens in a manner which is beyond praise. 



