11 



be recognised by its pale^ellow colour, with the gallo-nitrate mix- 

 ture, and must then be immediately transferred to clean blotting 

 paper, and all the moisture carefully removed from the surface. A 

 more even distribution of the gallo-nitrate solution may perhaps be 

 obtained by pouring a little out on a slab, and passing the iodized 

 paper over it, taking care that contact in every part is secured, and 

 blotting as before. To save time, the gallic acid may be applied 

 previously, and the paper kept thus half prepared. 



PLACING IN THE CAMERA. 



Having prepared the iodized paper as directed above, in which 

 state it is called calotype paper, it should be quickly transferred to 

 the camera frame, enclosed between a plate of slate or iron, and a 

 piece of plate glass to keep it smooth. If the slate or iron be 

 gently warmed, the sensibility of the paper will be increased. 

 The camera must now be put in the proper position, directed 

 towards the object to be copied, and a good clear picture obtained 

 on the ground glass. This picture, when an achromatic glass is 

 used, will give a good working focus ; but when the camera is fitted 

 with a miniscus, or any other kind of non-achromatic lens, a pecu- 

 liar adjustment is necessary to obtain what is called the chemical 

 focus, which differs materially from the optical or visible focus. 

 This chemical focus is about one thirty-sixth part shorter than the 

 other, but the scale should be adjusted according to the lens and 

 camera used. The frame, with the prepared paper, the shutter 

 being perfectly closed, is now placed in the camera. The time of 

 exposure here depends upon so many circumstances, the strength 

 of the light, the colour of the object, the description of lens used 

 in the camera, etc., etc., that it is impossible to give any practical 

 rules upon the subject, experience will be the best instructor. 

 With a single achromatic lens in the morning sunshine, from thirty 

 to sixty seconds is perhaps requisite for a building, and from one to 

 two minutes for a portrait : in the shade from two to three minutes 

 are required for either. Pictures are taken in a much shorter time, in 

 from ten to twenty seconds, by using a combination of lenses, or 

 with a single lens under very favourable circumstances. The best 

 position for taking a building, is at a distance about twice the 

 measure of its greatest dimension, and from an elevation of about 

 one.third of its height. Where some parts of the building are 

 nearer than others, place the focus to that part which it is most 



