Contagious Diseases of Insects. 273 



treated for study with the microscope. A droplet of the fluid 

 was allowed to flow from the tip of the pipette upon the 

 cover glass, spread in a thin film by means of the capillary 

 glass tube, and either placed at once upon the slide for imme- 

 diate examination, or laid aside under a glass shade to dry. 

 After drying, if it was desired to stain and permanently mount 

 the specimen, the cover with the film attached was passed 

 repeatedly through the flame of an alcohol lamp, covered for 

 some minutes with a drop of the staining fluid (the glycerine 

 aniline colors recommended by Prof. T. J. Burrill*), and then 

 thoroughly washed with distilled water. The covers thus pre- 

 pared were often mounted in balsam, but most frequently, at 

 first, in carbolized water in very shallow cells made with white 

 zinc cement, this cement being also used to fasten the covers 

 to the slides. For microscopic study of the material, my 

 principal reliance was a superb iV-inch homogeneous immersion 

 objective, made to order for the purpose by Herbert R. Spencer 

 & Company, of Geneva, New York. This objective was used 

 with Bulloch oculars, giving powers ranging from 500 to 1,450 

 diameters. Some of the more interesting or difficult slides were 

 also studied under a A-inch homogenous immersion of Zeiss. 



The measurements here reported were originally made by 

 means of an eye-piece micrometer so graduated that with the 

 highest powers used, each space equalled 2 /*, the micrococci 

 being commonly measured in doubles and chaplets. Many of 

 these measurements were verified by repetition with a more 

 finely divided micrometer, the spaces of which, with a power of 

 1,000 diameters, had a value of .3 ^5 but practice with the more 

 coarsely spaced scale enabled me to measure as accurately with 

 this as with the other, and with much greater convenience. 



The products and results of the fluid cultures were com- 

 monly so satisfactory that I rarely resorted to solid cultures 

 upon gelatine films. A few of these were made, however, but 

 not with the micrococci of the cabbage worm; and they will be 

 described under the head of the Datana larvae. As I was pri- 

 marily interested only in the disease and secondarily in the 

 bacteria, cultures on films were less essential to my purpose 



* Proceedings Amer. Society of Microscopists, 1883, p. 79. 



