DELINEATION OF MICROSCOPIC OBJECTS. 205 



process of fixing and hardening. After the image is fully deve- 

 loped, the plate is placed in a saturated solution of Hyposulphite 

 of Soda, or a dilute solution of Cyanide of Potassium, either of 

 which will dissolve out the silver salt which has not been acted on, 

 and leave the picture clear and transparent in those parts which 

 are ultimately to represent the dark parts of the object. When 

 taken from the Hypo, solution, the plate should be placed in a 

 strong solution of common Alum. This partially tans the Gela- 

 tine, renders it quite insoluble, and prevents any tendency to leave 

 the glass. After being well washed to remove the fixing and har- 

 dening salts, the plate may be dried, either by the fire, or by 

 spontaneous evaporation, and is then fit to produce a positive 

 picture either on glass as a lantern transparency, or on paper as an 

 ordinary photograph. 



The positive picture may be obtained in two ways : either by 

 setting up the negative, and obtaining, by transmitted light, a 

 positive image in the camera, then dealing with this image as 

 with the original negative, going through the same processes of 

 development and fixation ; or by placing a second dry plate in 

 close contact under the negative, and allowing the light of an ordi- 

 nary gas-burner to fall on it for about 5 seconds, after which it can 

 be developed, etc., as usual. The time of exposure will depend 

 on the intensity of the source of light, on the distance from the 

 source of light, and on the density or transparency of the nega- 

 tive ; but with an ordinary tulip burner, at a distance of about 

 five feet, and with a fairly clear negative, five seconds will be 

 found sufficient. 



If it be desired to produce a positive on paper, this can be best 

 done by using some of the papers which are now sold ready sen- 

 sitised, and which will keep, if protected from light, for an indefi- 

 nite period. This paper is exposed to ordinary day light under 

 the negative, and examined from time to time to note the progress 

 of the action of the light. When sufficiently dark, the paper is 

 removed, and placed in a solution of Hypo. Soda, which leaves the 

 print of a disagreeable red tint ; it is afterwards toned down in a 

 solution containing Gold Chloride, and when the desired tint is 

 obtained, should be well washed to remove all traces of the salts, 

 and dried either by the fire or by quiet evaporation. 



