950 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of a ruling-engine to the ^w ^^ i o o oV Tn7? ^^ *^ inch, is all that is 

 required to produce a ruling. I know that very many other conditions 

 must be satisfied to produce perfect rulings. 



" I do not believe that I will ever attempt to rule higher than 

 1,000,000 lines per inch, as from my jiractical experience and 

 judgment I have concluded that that is the limit of ruling." 



Whether 1,000,000 lines to an inch have really been ruled 

 must remain an unsolved problem, for, as Prof. Helmholtz has shown, 

 that number is very far beyond the limit of resolution. 



0. Collecting, Mounting, and Examining Objects, &c. 



Koch's New Method of Pure Cultivation of Bacteria.* — During 

 the recent meeting of the International Medical Congress Dr. Koch 

 explained the methods he had adopted in his researches into the 

 relation of Bacteria to disease, and amongst them was a new and 

 simple method of obtaining pure cultivations of the species of Bacteria, 

 which Prof. Lankester considers to be " likely to mark altogether a 

 new era in the study of the relations of Bacteria to certain diseases," 

 and the determination of what effects are due to one species of 

 Bacterium and what to another. To effect the separation of species 

 in a mixture, Mr. Lister employed a method of dilution and division,| 

 using a fluid as the nutrient medium of cultivation, as hitherto has 

 been the almost universal practice. This method is tedious and liable 

 to failure owing to the great care necessary to ensure and maintain 

 sterilization of the cultivation fluid whilst exposed for the purpose of 

 inoculation, and again for further examination. Dr. Koch's new 

 method of cultivation essentially consists in the substitution of a solid 

 for a fluid medium, and he was led to it by the use of the method, 

 known to all mycologists, of cultivation upon slices of potato or beet- 

 root. It is readily observed, when slices of boiled potato are exposed 

 in a damp condition to the atmosphere, that the surface of the slice 

 becomes the seat of development of various Bacteria and of moulds, 

 the spores of which fall from the atmosphere on to the exposed slice. 

 A fact which struck Dr. Koch as of importance in reference to the 

 slices of potato was this, that the various spores falling on to it 

 remain where they fall, and from the spot where each spore or germ 

 originally fell it proceeds to multiply, producing around it a sym- 

 metrical hemispherical growth of perfect purity. In fact, owing to 

 the solid character of the nourishing support, the germs and spores 

 cannot get mixed as they do in a liquid ; each remains distinct from 

 its neighbour, even though in very close proximity, and without any 

 trouble from the resulting growth which proceeds in a day or two 

 from each germ, new and perfectly pure cultivations may be started 

 in suitable sterilized fluids. 



Dr. Koch's method consists in substituting for the potato slice a 

 layer of gelatine, which is so saturated with water as just to become 

 solid on cooling. The gelatine liquid is readily sterilized by boiling, 

 and into it can be introduced either Pasteur's salts, peptones, blood- 



* Quart. Journ. Micr. Sci.. xxi. (1881) pp. 651-4. t Ibid., xviii. p. 191. 



