126 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Developing Photo-micrographs.* — The microscopist who occa- 

 sionally photographs his specimens finds that his developing solu- 

 tions deteriorate by keeping, and often when he comes to use them, 

 after standing untouched for some time, they do not act properly. 

 Especially is this true of developers containing pyrogallic acid, which, 

 as ordinarily made, soon lose their strength. It is customary to make 

 up the solutions and keep them ready for use, but owing to the cir- 

 cumstances above mentioned, this plan is not a good one for micro- 

 scopists who only use them occasionally. 



Mr. E. Hitchcock has adopted the following plan for developing, 

 which enables fresh solutions to be readily made without loss of time. 

 There should be always at hand citric acid and pyrogallic acid in 

 powder, strong ammonia ( ' 880), and a solution of potassium bromide, 

 50 grains to the ounce of water. When about to develop the plates, 

 dissolve 1 • 5 grains of citric acid in 8 ounces of water. In practice 

 it is not necessary to weigh out the exact quantity, as it can be 

 measured on the point of a knife, after a little experience. Then take 

 half a drachm of ammonia and mix it with 8 ounces of water. Go 

 into the dark room with the solutions, put the exposed plate into the 

 developing dish, and proceed as follows : for a 4 x 5 plate take 

 1 ounce of citric acid solution and add to it 2 grains of the pyro- 

 gallic acid in powder, measuring that quantity in the hand, or on a 

 spatula. It dissolves almost instantly. Then add one ounce of the 

 ammonia solution and a drop or two of the bromide, and flow the 

 whole over the plate. The development proceeds slowly, and may be 

 controlled in the usual manner by adding more bromide, or a few 

 drops of dilute ammonia, as the case may require. 



Action of a Diamond in Ruling Lines upon Glass.j — Prof. W. 

 A. Rogers writes, " In offering a communication upon the subject in- 

 dicated by the title of this paper, I am not unmindful of the fact that 

 I enter a field in which I acknowledge a master. Since the death of 

 the incomparable Nobert, Mr. Fasoldt, of Albany, stands easily first 

 in the art of fine ruling. I desire to repeat here the reply which for 

 the past three years I have invariably made to inquiries for test-plates 

 from my own machine — viz. that with Mr. Fasoldt's special facilities 

 for this class of work he can, I have no doubt, produce far better 

 results than it would be possible for me to obtain by chance efforts. 

 I have thought it better to confine my attention to another equally 

 important problem — viz. an attempt to obtain copies of the im- 

 perial yard and of the metre des archives, at the temperature at 

 which they are standard, to subdivide these units into aliquot parts 

 and then to obtain a microscopical unit whose subdivisions should 

 be so nearly equal that the Microscope would fail to reveal the 

 difference. The first part of this work has been mainly completed. 

 Two independently obtained copies of the imperial yard yield nearly 

 identical values for the length of this standard unit. Three indepen- 

 dent comparisons with the metre des archives agree within very 

 narrow limits in defining the absolute length of the metric unit, both 



* Amer. Mon. Micr. Journ.. iv. (1883) p. 198. 



t Proc. Amer. Soc. Micr., 6th Ann. Meeting, 1883, pp. 149-G5. 



