630 LABORATORY EXERCISES 



For this purpose a metal guide plate is screwed on to the inside of the lid, which 

 latter is reversed when the instrument is arranged for use and the marking apparatus 

 is placed on this plate. This apparatus consists of a vertical pillar with square base 

 ■ plate and a metal frame which is vertically adjustable by means of a rack and pinion. 

 The horizontel movement is obtained by moving the entire dish carrier along the 

 guide plate which is screwed on to the box lid. 



The Petri dish is secured in the frame by means of two milled heads which are 

 fixed on the right-hand side and at the bottom. 



Immediately behind the Petri dish is mounted a glass screen divided into squares, 

 which as a further aid to localization, are subdivided and numbered. 



A second pillar is screwed into the lid in front of the dish holder and carries the 

 lens. The lens is vertically adjustable and is threaded for focusing purposes. 



Below the lens carrier is fitted a horizontal bar which serves as a hand rest when 

 marking oS the colonies. 



A special counting screen is provided with fifteen square openings arranged in a 

 V-shape (echelon) by means of which the number of colonies at four places in sixty 

 squares may be determined. 



At the upper edge of the counting screen lines are ruled which serve as scales for 

 the Petri dish; the numbers on the one side indicate the diameters in millimeters 

 corresponding to each scale line, while the numbers on the other side indicate how 

 many times the area of the sixty squares is contained in the area of the whole Petri 

 dish. Thus in order to ascertain the total number of colonies in the dish, it is only 

 necessary to count the number of colonies in the sixty squares and to multiply the 

 figure thus obtained by the proportional number required by the diameter of the dish. 



. LESSON 26 



LABORATORY TVORK IN SYSTEMATIC BACTERIOLOGY 



As it is important for students in mycology to be able to identify the various 

 species of bacteria, which they may meet in their investigation of the fungi, the fol- 

 lowing suggestions are made as to the systematic study of the forms of bacterial 

 life. Ordinarily, where the other groups of fungi are to be considered, time will not 

 permit a detailed systematic study of the bacteria where cultural methods are re- 

 quired in the identification of the specific forms. Yet much can be done in the class- 

 room with the microscope in the study of the morphology of selected species. The 

 following exercises are presented as suggestions to the teacher and student of 

 mycology. 



First Exercise. — ^The teacher can distribute to each member of the class a selected 

 number of bacteria in culture tubes. Each tube should be numbered, so that the 

 student, after determining the generic character of the different organisms handed 

 to him, can attach the number to his specific determinations, so that the teacher 

 can check off the results of each student's work by the numbered list of species 

 kept for such classroom work. The bacteria from each of the culture tubes should be 

 mounted in balsam after staining with carbol fuchsin, or some other approved stain, 

 and kept for future reference and study. 



