252 HOW TO WORK 



time is effected if the objects be first selected for a suitable objective 

 and the mode of illumination arranged accordingly. 



For such objects as are of a more or less non-actinic colour, as 

 some entire insects, or their various parts, Dr. M. also tried a plan by 

 giving to the supporting slide a coloured transparent varnish of the 

 same tint, or by placing beneath the slide holder a parallel plate 

 of tinted glass chosen to suit as nearly as possible the necessity 

 of the case. But the best results were obtained by using a slow 

 collodion, a more acid bath, and giving a longer exposure, which 

 was done without fogging. Some of these results were exhibited 

 on the screen before the London Photographic Society in Dec. 

 1864, and the Microscopical Society in March, 1865. It should 

 be borne in mind that when any coloured medium is placed between 

 the mirror and objective, it has the most effect when placed at 

 the part where the light is least concentrated, and also that there 

 is no conversion of white light, but simply a transmission of the blue 

 and closely allied actinic rays when the ammonio-sulphate of copper 

 cell or blue glass is used ; hence the time of exposure must be 

 augmented. 



It is a desideratum to obtain the monochromatic and actinic 

 rays without having lost so much of their power by transmission 

 and absorption ; and it is just possible in the case of objects which 

 can be mounted in fluid, that such a medium may be found as 

 may enable us to employ the ordinary methods of illumination. I 

 believe Dr. Maddox is experimenting in this direction. It has also 

 been proposed to focus through a screen of polished parallel blue 

 glass, and to remove this when the sensitised plate is being impressed. 



Various media require different exposures under similar conditions 

 of illumination ; without a heliostat, rapidity in impression is neces- 

 sary to the most perfect definition. The refracting power of the 

 medium should correspond closely to the refracting power of the 

 object. 



The time of exposure varies from so many causes that it proves 

 one of the chief difficulties in photomicrography ; the distance of 

 the object from the screen, its colour, the medium in which it may 

 be mounted, the media through which the sun's rays are transmitted, 

 the nature of the first incident or reflecting surface the actinic 

 power of the sunlight, which varies considerably at different hours 

 in the day, the condition of the atmosphere, and the number of 

 lenses of which the objective is composed. This last operates greatly, 

 those in the high powers consisting of only three sets, and the first 

 a single front, as Mr. Wenham's, being the most rapid. Experience 

 and trial are the only guides to success. 



