1900-1901. | Orthochromatic Photography. 195 
affect the photographic film to a very considerable extent. 
Here is a photograph taken on an Imperial special-rapid plate 
of the spectrum of a Welsbach light (Plate XXIII. 1). You 
will see how closely it corresponds to the diagram just shown. 
The deposit does not extend beyond line F, where it abruptly 
terminates. The continuation beyond the visible violet rays 
is well seen. Now we can understand how it is that when a 
lady has her photograph taken, if she is wearing a nice blue 
blouse with white spots upon it, the whole garment shows 
as white, and the pleasing contrast which existed in the 
original is lost in the portrait. So again, a vase containing 
yellow daffodils and bluebells will show the bright yellow 
flowers as nearly black and the blue as nearly white in the 
photograph. Everybody knows how unsatisfactorily blue eyes 
are represented in portraits. From these instances we can 
see that although in many things we can rely upon the 
photographic plate to give us pretty nearly absolute truth, 
yet when we come to bright colours in the subject there is a 
great falling off. 
The outcome of the experiment of photographing the spec- 
trum shows us that for the correct rendering of pwre colours 
the ordinary photographic plate is practically useless. How is 
‘it, then, that we are able, as a rule, to obtain so many beautiful 
photographs in the ordinary way—photographs in which, as a 
rule, there is little that appears amiss? The reason is plain. 
In nature pure colours are rare, and from every object which 
we see there is so much of white light reflected, that all the 
colours being more or less mixed with it, they affect the photo- 
graphic plate in some degree ; but whenever a particular colour 
approaches purity, like the yellows, reds, or blues, for instance, 
then we are able to observe that in a photograph they are far 
from correctly rendered. 
Means have at length been found by which the colour defect 
in the photographic plate has been in a measure overcome. In 
considering the effect of the spectrum on an ordinary photo- 
graphic plate, we saw in the slide shown that the yellow and 
red rays produced practically no effect. But as we find in 
the spectrum that heat rays pervade all parts, though the 
great mass of them are found in the red and infra-red portion, 
and as we also find with regard to the visible rays that they too 
