214 THE PHOTO-MICROGRAPHY OF 



sarily produces rather thin negatives. With lower powers there is 

 apt to be so much very fine detail, and a longer exposure in pro- 

 portion to the amount of light, that the deposit of silver is greater, 

 and this process gives too much density, effacing some of the 

 details. 



II. — Pour over the plate a saturated solution of the red iodide 

 of mercury and potassium iodide, and then treat with potassium 

 sulphide. This process is perhaps the best for very thin negatives. 

 III. — Where the negative does not need very much more den- 

 sity, treating the plate with potassium sulphide alone until the film 

 is grey to the back will be sufficient.. 



The lower-power objectives I have found more useful in histo- 

 logical demonstrations than the higher, on account of their possess- 

 ing far more penetration and a flatter field. They also allow of a 

 greater length of bellows, equivalent to deeper eye-piecing, which 

 makes the details larger, and thus the field. To a certain extent 

 the size of the field is a matter of choice. I like to have it as 

 large as possible without straining the objective, so that the details 

 are easily seen. Objectives, being made to work at a particular 

 distance from the object, are more or less at a disadvantage when 

 that distance is increased or lessened. The longer the bellows, 

 the nearer the lens must approach the object, and there are limits 

 to this procedure which are soon reached. Uut the low powers 

 bear a long bellows without being at a great disadvantage much 

 better than the higher. This is sometimes convenient to make 

 use of. For example : by using a one-inch-and-a-half objective 

 and three feet twenty-six inches of camera bellows, a field, includ- 

 ing four times as much, and of the same degree of magnification, 

 and as clear as that from a quarter-inch objective, was gained. 

 The specimen was not one that tried the definition much — a 

 carmine injection of the blood-vessels in a rabbit's tongue. 



High powers allow of very slight or no departure from their 

 normal working distance, just as they do not bear deep eye-piecing. 

 But the chief disadvantage is in the lack of penetration and in 

 the great curvature of the field. 



For the above-mentioned reasons, I think it will be found best, 

 mth tissues, to use the lowest power compatible with the resolu- 

 tion of the necessary details, and to keep as near as possible to the 



