BAIBD dcTATLOCK. (LONDON) LTD. 



1011 



Neisser's Marking and Counting Apparatus for Bacterial Colonies. 



The Neisser counting apparatus provides the means of counting the bacterial colonies in cultures, etc., and 

 for marking particular colonies, for which purpose the vertical construction of the apparatus renders it very 

 convenient in use. 



The Petri dish is held firmly in a vertical frame by means of a spring lever and is thus brought exactly opposite 

 the eye of the observer. 



Immediately behind the Petri dish a glass screen, divided into squares, is mounted, and is ruled with sub- 

 divisions as a further aid to localisation. 



A short distance behind the frame containing the dish, a second carrier, fitted with a thin metal screen is 

 mounted, in order to reduce the transmitted light and allow of the more ready observation by lateral illumination. 



The whole frame carrying the dish is adjustable in both the vertical and horizontal directions by rack and 

 pinion, a focussing magnifier being mounted on a pivoted arm in front of the frame and permitting of the critical 

 examination of all parts of the dish. 



A rigid bar fixed to the lens holder at any point by a set screw, provides a support for the hand when marking 

 the colonies. . 



The complete mechanism is carried on a pillar, mounted in a heavy base. 



A special counting screen is supplied, provided with 15 square apertures arranged in a wedge pattern, and 

 serving to estimate the number of the colonies, at 4 places, in 60 squares. 



The upper edge of the counting screen is provided with lines which serve as scales for the Petri dish : the 

 numbers on the one side show the diameters of the latter in millimetres, whilst those on the other side indicate the 

 number of times the superficial area of the 60 squares is contained in that of the whole Petri dish. Thus it is 

 necessary only to count the colonies in the 60 squares and to multiply this number by the factor correspond ir.g 

 with the size of the dish in order to ascertain the total number of colonies in the dish. 



1011' Neisser Apparatus, with one lens magnifying 8 diameters 



each 700 



1012 1 Neisser Apparatus, with solid instead of hinged pillar, and with one lens magnifying 

 8 diameters . . . . each 



6 15 



CROSS STREET HATTCnsT GARDEN, E-C 



