126 



PHOTO-MICROGRAPHY 



Occasionally exactly the reverse may be desired : the operator may have taken 

 an excellent photograph of a difficult object — especially, perhaps, of a diatom, and 

 which would suit his requirements if it w^ere only a little more dense. To intensify 

 this is a simple process and there are many formula3 : here follows the one we use 

 and the method. The dried negative is thoroughly re-wetted and placed in a con- 

 centrated solution of perchloride of mercury. It turns completely white. When 

 white all the way through^ let it be well washed for half an hour, and then returned 

 to the developing dish, into which should be poured some of the hydroquinone 

 developer. The negative will turn rapidly black. When this is seen on looking 

 through the negative to be complete, more washing must be carried out for an hour. 

 Streaks and spots arise nearly always from too short and inadequate washing. If 

 it be desired a very dilute solution of ammonia may be used instead of the hydro- 

 quinone developer. We think for portraiture ammonia is the better ; but for 

 diatoms, bacteria, and other microscopical work we prefer the hydroquinone. The 

 operator should be careful not to allow the fluids to touch his fingers more than 

 possible, wearing finger tips if he can procure them, for a stain results which is most 

 difficult to remove. 



Let us now place a half-inch objective on the microscope and use it with what is 

 called a " projection " eye-piece. This eye-piece is made especially for projection 

 purposes, and can be procured for achromatics as well as apochromatics. It has a 

 considerably smaller field than the ordinary eye-piece, such being limited by a 

 diaphragm which is meant to be focussed on the ground glass by a slight turning of 

 the front lens. When this diaphra,gm is so focussed, the eye-piece performs at its best- 



Let us use the same slide of the proboscis, but try this time to get details of some 

 one or more of the smaller suctorial tubes. These should now be seen, if the objective 

 be a good one, very sharply defined on the ground glass, but it must be borne in mind 

 that the low-power condenser used at first should be removed and one of higher N. A. 

 substituted, otherwise the full benefit of the higher N. A. of the , objective may be 

 impaired. We must now, too, be more careful about centreing the condenser, the 

 limelight, and the auxiliary lens, for errors of no consequence when using the inch 

 objective are now of more importance. 



The exposure will have to be increased, and as the attention of the author has 

 been called to the fact that most writers afford little information upon the subject of 

 exposure, and as we desire to make what we have to say practical and instructive, 

 so we will try and make an attempt to be as explicit as possible. 



