146 



BACTERIA IN RELATION TO PLANT DISEASES. 



Fig. 130* 



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

 the picture. 



There are two other ways of making lantern-slides, i. <?., 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, i. e., 

 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 

 I that various lenses may be accommodated and so 

 I that lantern-slides maybe made from large negatives 

 if desired, i. <?., 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 1 1 by 14 plates. When making lantern- 

 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, e. g., with stop 64 u. s. from 

 YZ second or less in bright light to fifteen minutes or more in very 

 dull light with dense negatives. L,antern-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. 



*' * 



*FiG. 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. 



