134 



BACTERIA IN RELATION TO PLANT DISEASES. 



magnification ; No. 5, the largest field ; 

 No. 3 will give a sharp image of a flat 

 object a centimeter in diameter. Special 

 condensing lenses are required. These 

 fit into the substage in place of the 

 Abbe condenser. One condenser serves 

 for Nos. i, 2, and 3, and another for Nos. 

 4 and 5 (fig. 122). 



In photographing ponred-plate colo- 

 nies natural size, there are several ways. 

 It may be done by reflected light, as 

 shown in fig. 123, in which case the 

 colonies sometimes cast deep shadows. 

 Such shadows may be avoided by 

 mounting the camera as shown in fig. 

 124 and gently twirling it during the 

 exposure. The Petri dish may also be 

 photographed by transmitted light ex- 

 actly as if it were a negative for a 

 lantern slide. The Petri dish is then 

 held in place in the darkened window 

 or in front of the camera box by crowd- 

 ing it into a hole cut in a square of 

 thick leather, paper, or sheet-rubber 

 (1 inch), which is then fastened over 

 the kit or framework by eight thumb- 

 tacks, or, better, it may be held in place 

 by two stout rubber bands, as shown in 

 the photographs (plate 17 and fig. 125). 

 With stop 32 11. s. and Seed's 2J-X 

 plates the right exposure in Washing- 

 ton is usually somewhere between 

 ^ second and ^ second in sunny weather 

 and 3 to 5 seconds in cloudy weather, 

 using a Voigtlaender collinear lens, 

 series III, No. 6, and south light. 



Atkinson gets very good results by 



Fig. 124.* 



*Fic. 124. Modified Collins-Brown camera swung from the ceiling and set to magnify about 

 X i?4. The four suspending strings, which are of very strong fish-line, end in an S-shaped hook, 

 the upper end of which hooks over a ring attached to a stout cord pendant from the ceiling. The 

 length of bellows in this camera as modified by the writer is 25 inches. The lens used with it is a 

 Zeiss Double-Protar, Series Vila. No. 13, made by Bausch & Lomb, Rochester, N. Y. This is the 

 type of lens known also as the Zeiss Convertible Double Anastigmatic. This lens has a focal dis- 

 tance of pJ4 inches, or, when only the front or back half is used, 165/2 inches (16 according to Zeiss 

 catalogue). It is provided with a Bausch & Lomb No. 2 Volute shutter. A cork support was 

 placed under the object carrier to steady the apparatus while it was being photographed, but in 

 actual use the camera swings free, and if one desires to avoid shadows the apparatus is given a 

 gentle twirl just as the exposure begins. The object carrier is easily removed, and is held in place 

 at any level by two set-screws. 



