and Laboratory Methods. 1201 



The fixing bath, while by no means as important as the developing solution, 

 may have its value underestimated. It is not uncommon to find operators who 

 mix up water and hypo in almost any proportion and, without filtering the solu- 

 tion, use it until it becomes so discolored as to afi^ect the film of the plate inju- 

 riously. A little care will greatly economise the use of the hypo and at the same 

 time produce much better negatives. I find the following bath entirely satisfac- 

 tory, cheap and convenient: Prepare and filter saturated aqueous solutions of 

 hypo and boric acid. Mix one part of the hypo with three of the boric acid 

 solution. This bath will keep until the hypo is exhausted without discoloring 

 and, being acid, hardens the film. 



Plate and developer are important agents in the production of a good nega- 

 tive, but, without the proper adjustment of light effect to the speed of the plate, 

 they are worthless. The exposure must be judiciously regulated so that the 

 darkest parts of the object will not be allowed to produce any effect upon the 

 sensitive film, while, at the same time, the light must be allowed to act long 

 enough to be effective, in various degrees, over the lighter portions of the object. 

 General instructions regarding this part of the work are of little value, so I will 

 outline the actual conditions under which negatives have been produced. 



Source of illumination- — the crater of an arc light placed at a distance of 

 two feet from the object and having interposed between it and the condenser a 

 ground glass disc. This disc stands about six inches from the crater and its 

 matt surface is made somewhat transparent by rubbing with glycerin. 



Condenser — a parachromatic of 1.30 N. A. in homogeneous contact with 

 the lower portion of the slide. The diaphragm registers a numerical aperture 

 of .5. 



Object — a section of embryonic tissue 6^^ micra thick, stained with iron- 

 haematoxylin and mounted in balsam. 



Objective — a '2 mm. homogeneous immersion apochromatic of 1.30 N. A. 



Ocular — a No. 2 projection. 



Magnification — 1000 diameters. 



Plate — a " Carbutt Ortho. Process." 



Under these circumstances, an exposure of 20-30 seconds is sufficient. 



Two things are to be guarded against during the time of exposure ; viz., 

 flickering of the light and vibration of the microscope or camera. Either of 

 these untoward circumstances will ruin what might otherwise prove to be a good 

 negative. 



With the proper exposure and by the use of the metol-hydrochinone devel- 

 oper, the image will begin to appear upon the plate in about 30 seconds, and develop- 

 ment will be complete in about five minutes. In the " Process " plate, very little 

 of the image will show upon the reverse, or glass, side of the plate. The prog- 

 ress of the development is best observed by examining the image under trans- 

 mitted light. Somewhat greater density than is finally desired should appear, 

 since some of it is lost in the hypo. 



Fix until all the unreduced silver salts are removed, and the shadows are 

 clear. To be sure of this, allow the hypo to act some minutes after the last trace 

 of milkiness has disappeared from the film. 



