MICROSCOPIC OBJECTS. 



19 



found very useful, but all intensification 

 should be avoided if possible. Look for 

 good results to the development alone, and 

 let intensification be merely a last resource. 



Other intensifiers are potassium sulphide 

 and ammonium sulphide ; either salt may 

 be made up to the strength of I dram to 

 20 ounces of water. 



One of the best silver intensifiers is that 

 given by Mr. W. Brooks, and is as follows : 

 "After the plate has been well washed 

 from the hypo, place it in a weak solution 

 of alum and citric acid solution : 



Stock Solution. 



Saturated solution of alum, 10 ounces. 



Citric acid, i ounce. 



For the solution above named, I dilute one 

 part to four of water, allow the plate to re 

 main in it about five or six minutes ; in the 

 meantime, place in a developing cup about 

 (say for a small plate) two drams of the 

 stock solution of alum and citric, and place 

 in it about four grains of pyro. ; when dis- 

 solved, add a few drops of about a twenty- 

 grain nitrate of silver solution." 



This solution is to be applied to the plate 

 until sufficient density be obtained ; when 

 the plate is washed, it is placed in thehypo_ 

 bath for a few minutes, again washed, and 

 finally treated with alum and citric acid so- 

 lution to clear it. Plates thus intensified 

 should not first be dried. 



10. Too Great Density. — This is more 

 frequently met with when using the plain 

 pyro. developer. Perhaps the simplest 

 agent for removing it is Mr. Howard Far- 

 mer's fenicyanide reducer given above in 

 speaking of green fog. The plate, if dry, 

 is soaked in water for a few minutes, then 

 placed in the solution, and examined from 

 time to time until sufficiently thin. Only a 

 few drops of ferricyanide must be added, 

 or the action will be too rapid. Then wash 

 well in running water, and dry. 



I I. Shrinkijig of the Gelatifte. — This 

 may arise from the gelatine being too soft, 

 or from heat being employed to hasten the 

 drying, which is a great mistake. The 

 shrinking may be slight, spoiling the fine 

 niicioscopic detail, or rhay amount to actual 



distortion of the image. Either way, the 

 negative is ruined ; the photo-micrographer 

 is therefore advised to use only plates pre- 

 pared with hard gelatine. Since the use of 

 the alum bath has become more general, 

 frilling is not so common as formerly, and 

 the photo-micrographer need hardly be cau- 

 tioned against it. Still, to avoid this, as 

 well as the more serious evil of shrinking of 

 the film, any plates found to be prepared 

 with soft gelatine should be rejected for 

 microscopic work. 



Lesson VIL — Printing. 



The advice given in the introduction is 

 here repeated : let the photo-micrographer 

 make all his own silver prints, as in no 

 other way can he hope for lesults of a satis- 

 factory nature. The process is rot diffi- 

 cult, and, when once mastered, the micro- 

 scopist will have the pleasure of seeing his 

 pictures real representations of the origin- 

 als, instead of being little else than carica- 

 tures, which is often the case when the neg- 

 atives are entrusted to anotiier to print. A 

 professional photographer may be able to 

 produce perfect specimens of art from por- 

 trait or landscape negatives, yet fail en- 

 tirely when he tries to print a photo-micro- 

 graph, simply from not understanding the 

 nature of the object represented. 



Ready-sensitized paper may now be pro- 

 cured of great excellence at a moderate 

 price. The photo-micrographer should 

 purchase the best obtainable, for the best 

 costs at first very little more than the worst, 

 and in the end costs really less, as there 

 will be no defective sheets to reject, to say 

 nothing of the superior quality of prints to 

 be produced on good paper. 



Frequently a photo-micrographic nega- 

 tive may require some little preparation be- 

 fore it is ready to print. For instance, di- 

 atoms being, as a rule, very transparent ob- 

 jects, require a short exposure. This ex- 

 posure, though sufficient to bri.ng out the 

 detail in the diatom, is not long enough 

 to give proper density to the background, 

 which should appear white in the finished 

 print. In this case the best plan is to 

 "paint out" the background, at the back 



