June 28, 1918] 



SCIENCE 



639 



spects that of the actual socl solution, should 

 be the first consideration. 



To the soil bacteriologist the solution ob- 

 tained under great pressures would be of doubt- 

 ful value. Many bacteria are destroyed by 

 high pressures (25,000 to 100,000 pounds). 

 In fact high pressures alone have been em- 

 ployed successfully in the sterilization of 

 fruits and vegetables." Studies of the micro- 

 organisms surviving these enormous pressures 

 would be probably only a matter of curiosity 

 and of no immediate value or utility. 



It seems that Dr. Lipman should have made 

 a thorough comparative study of the soil solu- 

 tion obtained from the same soils by the two 

 methods imder discussion before he could be 

 justified in making the statements set forth 

 in his preliminary article. 



Zae Northrup 



Bacteriolooic.\l Laboratory, 

 Michigan Agricultural College, 

 East Lansing, Mich. 



DRAWINGS ON LANTERN SLIDES 



Professor Gunthorp'.s letter in Science for 

 April 12 in regard to drawings on lantern 

 slides seemed to the writer to be an attempt 

 to solve the problem of writing upon clear 

 glass when the ordinary coated slides were not 

 available. The letter by Mr. Benton in the 

 issue for May 17 goes further into the solution 

 of this problem, and the suggestion of using 

 india ink is a good one, but the idea of past- 

 ing paper to a slide to secure a purchase for 

 the foot of a compass would lead one to sus- 

 pect that the use of coated glass had not been 

 tried. Even though this supposition is in 

 error the use of ordinary unexposed lantern 

 slides, fixed in the dark room, or of old slides 

 reduced by successive immersions in " hypo " 

 and Farmer's solution, may be new to some 

 and is worthy of mention. The transparency 

 of the prepared slide is all but perfect, the 

 coated side can be wTitten, drawn or ruled 

 upon at will, areas can be shaded or colored, 



5 Hite, B. H., Giddinga, N. J., and Weakley, 

 Chas. E., Jr., "The Effect of Pressure on Certain 

 Microorganisms Encountered in the Preservation 

 of Fruits and Vegetables," West Virginia Station 

 Bulletin 146, 1914. 



errors can be removed by the simple expedient 

 of scratching away the gelatin (and the re- 

 maining scar is surprisingly insignificant when 

 tlie slide is thrown upon the screen) and di- 

 viders or compass can bo used without danger 

 of slipping. In writing, the finer and firmer 

 the point, and the less ink, the better, as a 

 thick line will crack up into a mosaic; and 

 experience has shown that ordinary fountain 

 pen ink is much less liable than india ink to 

 crack in this way or to " ball " at the ends of 

 the strokes. Waterproof inks must be used, 

 however, if the slides are to be wet. If it is 

 desired to render large areas opaque, and it is 

 imiiracticable to use successive thin coatings, 

 cover them with india ink, preferably using a 

 brush, and when the surfaces have dried and 

 cracked cover them again. 



In coloring slides drawing inks may be used 

 and tlie surfaces so colored will not crack if 

 the ink is applied in thin enough coats. Hig- 

 gins's carmine will be found less suitable than 

 the other reds because of its heaviness of body 

 and rapidity of drying. A simple method of 

 improving one's chances of securing a smooth 

 result, however, is to soak the drafted slide in 

 water and then allow it to dry until there is 

 no free moisture present, until it is sticky, 

 before the colors are applied. If these pre- 

 cautions are taken and the wash is not too 

 thick an even uncracked surface will result. 

 Water colors, especially the stains and " lakes," 

 are highly transparent and generally preferable 

 to many of the drawing inks. For blended 

 outlines the colors should be put on while the 

 slides are covered with water in the customary 

 way, but for the sharp outlines which will 

 usually be desired in drafted slides the latter 

 should be approximately or entirely dry. 



Lantern slides prepared in this way need not 

 be covered to preserve the writings or figures 

 from abrasion, always a troublesome feature 

 when clear glass is used. Fingermarks will 

 show, though a slide pinched between the 

 fingers will take a mark more readily upon the 

 clear than upon the coated side, but these can 

 be removed from the latter, almost irrespective 

 of the ink or coloring materials used, by wash- 

 ing with pure alcohol. The ounce of pre- 



