38 



PHOTO-MICROGRAPHY 



prepared to admit the difficulty is great, but it can be very often got over to a great 

 extent in the following manner : — Consider the circumstances : a yellow interceptive 

 background from the colour of the mounting medium, which increases the exposure, 

 and a brilliant proboscis, which has only absorbed enough of the yellow mountant to 

 clear it and make it more transparent. An increase of exposure has been found to 

 increase the density of the background, it is true, but at the same time to cause over- 

 exposure of the proboscis itself. The only way we know of is to use a glass screen of 

 some colour to render the contrast in exposure less severe. A green pot glass, about 

 one- eighth of an inch thick and of medium density, will increase the total exposure, 

 it is true, some five or six times, but will often produce a more equally balanced 

 negative. Sometimes we should prefer a yellow screen, and with certain specimens 

 obtain a still better result. Let the photographer try the effect of each colour and 

 judge for himself When the contrast is not pronoimced enough, as often found with 

 specimens of bacteria, exactly the opposite treatment is required, and a glass is used 

 to increase it. 



The lantern plate should be washed after leaving the hypo for at least an hour, 

 and then be dried on the hot water tank, after having wiped the back. We do not 

 find wiping the front of any lantern plate a good thing to do. It is very useful in 

 the case of negatives, but with lantern plates certain markings sometimes occur if the 

 fabric be too severely pressed on to the emulsion. 



We should like to mention one more matter, which gave rise in our hands to a 

 large amount of trouble and experiment, and which will occasionally occur in warm 

 weather with these and other lantern plates. We refer to a distinct and fatally 

 pronounced yellowing of the clear portions of the film, M^hich shows itself after fixing. 

 It seems to resist all treatment, and it was a long time before we could ascertain the 

 cause. At first, common hypo was thought to be the cause, and that the yellowness 

 was only due to imperfect fixation. This was ascertained to be an error, and all the 

 solutions were alike called to book for the trouble. The emulsion was then blamed, 

 and the manufacturers called upon to explain. After a very considerable time was 

 spent, and after the most courteous attention of the Paget Company was well-nigh 

 exhausted, the trouble w^as discovered to arise from a staining by the developer when 

 the process was carried on too slowly, and to an insufficient subsequent washing 

 before fixing. We never get the trouble in winter, but even now, though rarely, come 

 face to face with it in summer. Considerable soaking in water after development 

 will remove it, but it is better to expose a little more, risking a browning of the final 

 image, which will occur from over-exposure with lantern plates as with Nikko paper. 



