MICROSCOPIC OBJECTS. 



minutes; no fear need he entertained of 

 the hyposulphite solution weakening the 

 negative. Then put the plate in running 

 water for half an hour. This will be suffi- 

 cient, but any trace of hypo, will be got rid 

 of in the next bath : 



Alum, 2 ounces. 



Citric acid, .... i ounce. 



Water, 20 ounces. 



This solution will also harden the film, and 

 render it less liable to injury from scratches 

 or wet. The final immersion in alum should 

 be regarded as absolutely necessary. It 

 will brighten up the negative, removing any 

 stains which the developer may have left, 

 and make it "quicker printing." 



The negative should not be dried too 

 rapidly. In fine dry weather it can be best 

 dried by placing it out of doors. In win- 

 ter time, the writer stands his negatives on 

 a warm mantel-piece, w here they dry in one 

 or two hours. The only precaution to ob- 

 serve is, that the plate should have previ- 

 ously been soaked in the alum bath for at 

 least five minutes; otherwise, if the film be 

 composed of soft gelatine, the heat of the 

 mantel- piece is apt to melt it. 



Lesso.n VI. — Defkcts in the Negative. 



As the defects in photo-micrographic neg 

 atives are very numerous, it may be well to 

 mention the chief, and their remedies, when 

 such exist. 



I. Unequal Tlbimination.— This is very 

 apt to occur when using very oblique light, 

 but may hajipen also with central light, 

 from improper arrangement of condensers, 

 etc. This defect may be known by the neg- 

 ative being dense on one side of the plate 

 and thin on the other. Do not blame the 

 plate maker for improperly levelling his 

 plates; this defect sometimes occurs, but 

 very rarely. If the difference of density on 

 each side is not very marked, it may be reme- 

 died by using matt varnish on the thin s-ide of 

 the plate, to diffuse the light when prii ting. 

 Adding a little yellow d)e to the varnish 

 often improves the result, but in all cases 

 the rough edge of the varnish should be 

 softened by the use ( f a little alcohol or 

 ether, or a na'ty mark will be left on the 



print, just under the boundary line of the 

 varnish. 



2. Too Pozverful Jllumination. — In this 

 case the object is "drowned in light," and 

 the picture comes out flat and degraded. 

 Remedy : take another negative. 



3. Reflection from the Apparatus. — 

 When the tube of the microscope is not 

 lined with cloth or velvet, a bright central 

 spot may often be seen on the screen whi'e 

 focussing, and a corresponding black patch 

 will be found on the negative, which will be 

 worthless. When using the eye-piece this 

 defect will not be met with. Reflection 

 from the camera will also ruin the negative ; 

 also using the eye piece without the cap. 

 Let the inside of the camera and microscope 

 tube be a dead black. 



4. Access of Stray Light to the Plate — 

 Probably through the connection of camera 

 and microscope not being light tight. Re- 

 sult — general fog. Use a thick black velvet 

 hood to connect the microscope with the cam- 

 era, and keep it in place with elastic bands. 



5. Green Fog. — This may arise from 

 the use of impure sodic sulphite, or, in the 

 plain pyro. developer, from using too much 

 ammonia. Green fog appears to be a sil- 

 ver deposit, from the fact that certain silver 

 Solvents get rid of it at once. Bichromate 

 of potash, or peroxide of hydrogen, may be 

 used, but the writer recommends the fol- 

 lowing', which is given by Mr. Howard 

 Farmer in the Year- Book for 1884 : 



A. — Potassium ferricyanide, i ounce. 



Water, 20 ounces. 



B. - — -Sodium hyposulphite, i ounce. 



Water, 20 ounces. 



First wet the negative if it has been dried, 

 pour a little of the hypo, solution in a cup 

 and add a few drops of the ferricyanide so- 

 lution. Dip a plug of cotton-wool in this 

 mixture, and sponge the negative rapidly 

 with it ; then plunge it in water, and wash 

 well. All trace c f green fog will have dis- 

 appeared. 



6. Over-exposure. — This should be con- 

 trolled in the development. After-intensifi- 

 cation rarely produces even tolerable nega- 

 tives from over-exposed plates. If much 



