PRACTICAL PHOTOMICROGRAPHY. 151 



"Followed after washing by fresh solution of hypo." (See 

 page 63.) 



GELATINE BROMIDE PLATES FOR SLIDES. 



In this process we have certainly the quickest method of 

 making slides for the lantern, but it can only be called the best 

 method when worked with care and skill. If the Reader 

 proposes to make all his lantern-slides " by contact," he had 

 probably better use chloride plates ; but if he objects to the 

 wet-plate process, he will find the process we now describe the 

 best for his purpose if he has to reduce or copy in the camera ; 

 while the gelatine bromide process may be made to answer 

 admirably for contact printing. Ninety per cent, of the 

 author's slides, reduced and contact-printed, are made with 

 commercial gelatine-bromide plates. 



The exposure required in the camera is from three to ten 

 times less than with wet collodion, the difference of ratios 

 depending upon the sensitiveness of the gelatine plates and 

 the quality spectroscopically of the light. In a very u actinic" 

 light the difference is minimized, in a " poor " light as regards 

 blue and violet rays the gelatine has a greater " pull." The 

 bromide plates are many times more rapid than the chloride 

 plates, not less, perhaps, than 20 times. 



Exposure is made in the usual way, but the light of the 

 " dark-room " must be much more subdued or non-actinic than 

 for collodion or chloride plates. This fact is too often disre- 

 garded. 



The developers we recommend are ferrous oxalate and 

 hydroquinone, but alkaline pyro is also available. 



The ferrous oxalate developer may consist of one part of the 

 saturated ferrous sulphate solution (page 03), to four, five, 

 or six parts of the potassic oxalate solution on the same page. 

 We always add half a grain of soluble bromide to each ounce 

 of the mixed developer. When this developer is applied the 

 image should begin to appear after about thirty seconds,, not 

 before twenty seconds ; density and detail should increase 

 steadily up to about four minutes ; when the development is 

 complete the knage seen by transmitted light ought to be 



