1900-1901. | Orthochromatic Photography. 201 
_ way the sky would be so much over-exposed that the beautiful 
— elouds would be entirely lost; but if an isochromatic plate be 
used with a very light tinted ray-filter, the blue of the sky 
- will be sufficiently held back for the clouds to make a fair 
impression on the plate. As a rule, the more colour in the 
subject the deeper the tint required in the ray-filter. In 
' working by gaslight, as in taking microscopic photographs, 
the ray-filter is seldom required, because the light is already 
yellow, and if a ray-filter is used it need be only of medium 
tint. All these artifices will soon occur to those who prac- 
tise orthochromatic photography for a little while and care- 
fully examine their results. The length of exposure must be 
_ found by practice, and it is difficult to give directions. But 
with the use of an exposure meter “mixed with brains” 
very few failures need be feared. Light-filters can easily be 
home made, but I do not recommend them, for this reason: 
it is not easy to get the two surfaces perfectly flat and parallel, 
and unless this is the case distortion, if only in a small 
_ degree, must result, and it would be a pity to interfere in any 
way with the beautiful working of the excellent modern 
lenses now in the hand of the photographer. The light- 
filters can be placed either in front of the lens or at the back. 
_ Personally I prefer the front position, and have my ray-filters 
cut in circular shape, mounted in cardboard, and of such a size 
that they slip easily into the hood of the lens, where they are 
secured by a light spring made of wire. Where the lens has 
no hood they can be made in the form of a cap to fit in front. 
We have now all that is required for the work, and it only 
_Yemains to remark in this connection that the development 
of orthochromatic plates needs some care. It must be borne 
in mind that they are sensitive to rays which do not affect 
the ordinary plate. It will not do to develop them in the 
_ full light of the dark-room lamp, or it will soon be found that 
_they have a decided tendency to fog. Even in putting the 
_ plates into the dark slide, it is best to let no direct light of 
any kind reach them. In my own practice, when developing, 
I use just enough light to see that the developer is properly 
flowed over the plate, and immediately withdraw the dish so 
_ that no light of any kind can reach it. To watch the appear- 
ance of the image the tray is brought to the light for exam- 
