bb PRACTICAL PHOTO-MICROGRAPHY. 



with the coarsest grained image the writer ever saw produced 

 by gelatine bromide emulsion, show any grain due to the 

 coarseness of the image deposit. We have, in fact, seen nega- 

 tives in most sensitive gelatine emulsion enlarged 5 diameters 

 without the slightest appearance of grain. The argument for 

 the wet-plate-and-camera-enlargement is that the ultimate re- 

 sult shows greater " penetrative " effect, that is to say that 

 there is less apparent difference of sharpness in different planes 

 of the object ; the writer made a large series of experiments 

 on diatoms, enlarging always to X 300, by direct micrographic 

 means, and by camera enlargement from original negatives at 

 100 and 150 diameters ; in every case the direct amplification 

 was superior to the "enlarged" negative; and the plates used 

 for the 100 and 150 diameter negatives were some wet, some 

 very slow gelatine emulsion, while the negatives direct at 300 

 were in every case an exceedingly rapid gelatine emulsion. 

 Thus much for practice, the theory is too intricate to follow 

 here. At one time the writer thought that wet collodion 

 would prove superior to gelatine emulsion, and he had got the 

 length of producing a considerable number of wet plate nega- 

 tives ; but on using slow gelatine plates on the very same sub- 

 ject, which were chiefly flies' tongues, and minute hairs, he 

 found the gelatine results were in every way equal to the col- 

 lodion, while the ease and certainty of the former process 

 were incomparably greater than with the latter, though the 

 writer is well accustomed to the wet plate process. 



But it is most important to make a wise selection of the gela- 

 tine plates to be used. Our objects may be divided into two 

 great classes : the coarse and the delicate. When our objects 

 are by nature coarse in detail, or when they are made actinic- 

 ally coarse by staining, we shall have no difficulty in getting 

 contrast in our photographs ; but when our objects are very 

 minute, or composed of very minute details, or when they are 

 so stained as to present very little contrast to the background and 

 between each other, the affair is quite different, and a quite differ- 

 ent class of plate is required. Where the danger is over-contrast 

 rather than want of contrast, a u thin ? ' plate, such as is com- 

 monly used in portraiture, is the best to use ; but in cases of 



