146 



BACTERIA IN RELATION TO PLANT DISEASES. 



Fig. 130* 



sharp as possible with stop wide open and then stops clown to 16 u. s. before making 

 the picture. 



There are two other ways of making lantern-slides, /. e., by contact exposure, 

 the gelatin films face to face, and by means of a long box-camera with the negative 

 in one end, the lantern-slide carrier in the other end, and the lens between the two, /. r ., 

 inside the camera-box, held in a framework sliding between the two ends and having 



front and rear bellows attached to its outside parts. 

 The method by contact exposure is not very satis- 

 factory unless the negative and the lantern-slide are 

 of the same size. The box-method is a very good 

 one. A box of this kind is very convenient, and 

 may also be used for enlargements up to 5 or 6 

 times. The bellows-extension should be ample, so 

 that various lenses may be accommodated and so 

 that lantern-slides maybe made from large negatives 

 if desired, i. e., the solid framework or track on 

 which the parts slide should be about 6 feet long, 

 and the bellows -extension to either side of the 

 middle piece should be not less than 3 feet, exclusive of the woodwork at each end 

 and in the middle. A very good apparatus of this sort is shown in plate 17. It is 

 the Folmer & Schwing enlarging, reducing, and copying camera, 

 mounted on a plain wooden table of home construction, and the 

 only defect I have discovered in it is that it has too short a bellows 

 for use with lenses having a 1 2-inch focus. It has a very neat device 

 for obtaining a sharp focus and many other conveniences, and might 

 just as well be made with a longer bellows. It is convenient to 

 have a box which will take n by 14 plates. When making lanteni- 

 slides the end of the box carrying the negative is pointed toward 

 the window and is elevated a foot or more to secure uniform light- 

 ing. The writer has found the Voigtlaender collinear lens, series III, 

 No. 6, very satisfactory for making lantern-slides and enlargements. 

 In plate 17 the bellows-extension used when making lantern-slides 

 from large negatives lies on the floor. 



The time of exposure for lantern-slides varies greatly with quality 

 of light and density of negatives, c. g., with stop 64 u. s. from 

 l / 2 second or less in bright light to fifteen minutes or more in very Flg ' 131 '^ 



dull light with dense negatives. Lantern-slides should not be developed with pyro 

 because it stains, and should not be developed with metol-hydro because it often 

 gives a foggy appearance if the contrasts in the negative are great or the exposure 

 is a little too long. Hydrochinon gives very satisfactory slides. 



*Fic. 130. Cross-level, made by The L- S. Starrett Company, Athol, Mass. Nearly actual 

 size. This is very convenient for use with cameras. 



fFio. 131. Device for cutting out light in air-shaft of dark-room. Diameter, 12 inches. 



