PRACTICAL PHOTOMICROGRAPHY. 109 



Mr. Wellington recommends an alcoholic solution of turmeric, 

 and a solution of picric acid will also be found to be of great 

 service. In any case the scraens must, when tested by aid of 

 a spectroscope, cut off some portion more or less of the violet 

 and blue of the spectrum. 



In cases where we have blue or violet lacking actinic contrast 

 with the white of the background, or likely to be over-exposed 

 before other colors (as yellow or red) can be sufficiently exposed, 

 we cut off some blue or violet by use of our screen, and the 

 paler is our violet or blue the darker must be our screen. But 

 if our violet be very dark, and more particularly if it be a 

 reddish violet, such as logwood shows at times, we shall get a 

 better result in presence of a red or yellow-brown contrast 

 stain, if the latter be pale, by omitting the screen entirely, 

 for the depth of the violet amounts in practice to so much 

 opacity, not to mention the red impurity of the violet stain. 

 But if the contrast red or yellow be also dark the screen may 

 be required unless our plate be very much corrected for color. 



It is not an uncommon occurence to find preparations very 

 faintly stained with red, and it is still more common to find 

 red stains and also yellow fade after a time. In such a case 

 there is want of actinic contrast between object and ground, 

 and the latter is exposed practically as fully as the former, and 

 so we can not get a white ground in our positive. In such a 

 case a rather dark yellow screen used either with an ordinary 

 or an orthochromatic plate will prove of great service. 

 Bacteriological preparations seem to be specially subject to 

 this fading, and we have many times got good results with a 

 yellow screen after repeated failures without one. 



Insect preparations and others similar, where we have yellows 

 approaching opacity at times are very difficult. Sometimes 

 the best result is got by using an ordinary plate without any 

 screen, sometimes a yellow-sensitive plate without screen, some- 

 times the latter plate with a screen added, sometimes a cobalt 

 blue screen, and sometimes a signal green. The best procedure 

 depends upon the depth and quality of the yellow in the object. 

 The common flea as usually mounted seems to come out best 

 with an ordinary plate, but we possess one which curiously 

 enough requires not only a yellow-sensitive plate which pro- 



