BLUE-PRINTING DIRECTLY FROM AGAR PLATES 



JEAN BROADHURST 



Teachers College, Columbia University 



Received for publication September 8, 1917 



This spring in planning, for teaching purposes, a large number 

 of illustrative series of photographs showing comparative bac- 

 terial conditions (boiled and unboiled water, effects of disinfec- 

 tants, milk kept at different temperatures, etc.) we found pho- 

 tographs as expensive as permanent glycerine-agar plates them- 

 selves. With amateurs the results are most uncertain, even 

 when a really good camera can be secured; and with the delays 

 incident to developing and printing the Petri dishes often spoil 

 or change materially before new exposures can be made. 



We finally hit upon the expedient of using blue-print paper, 

 printing directly from the Petri dish, and omitting the camera 

 entirely. The Petri-dish was placed flat upon the blue-print 

 paper and then exposed to the direct rays of the sun — a method 

 long used for leaf-prints. Occasionally the more transparent 

 colonies were ''touched up" by using a fine-tipped brush to 

 apply thick white paint to the under side of the Petri dish. 

 Unless the agar layer is thin and unless the light rays fall at 

 right angles to the dish, ''touched up" colonies appear blurred 

 in the prints. 



The blue print paper used by architects is sensitive enough for 

 mo^t purposes, and much cheaper than the kind sold by photo- 

 graphic supply houses. We had the large sheets cut into 

 5-inch squares, the total cost (including the cutting) totalling 

 but $L65 for 500 squares. 



A large board was used for holding the Petri-dishes. To the 

 board were fastened by screws or matting tacks narrow strips of 

 metal (old jig-saw blades, in this case). When these were lifted 

 or swung into position (2 or 3 to a dish) their free tips pressed 



187 



