54 



Journal of Applied Microscopy. 



square centimeters respectively, the 

 formula 7ri*- = area was used. 



In order to show the application of the 

 formula, the radius of the circle, whose 

 area is equal to ten square centimeters, 

 will be found from the formula as fol- 

 lows: 



7r = 3.1416 



7TV^ =r 10 or r~ = 10 -f- TT 

 10 -J- 3.1416 = 3,18309 or r^ 

 V3. 18309 = 1.78+ or r 



1.78 -f centimeters equals the radius 

 of a circle whose area is ten square cen- 

 timeters. Dividing the circle into ten 

 equal sectors, each sector has an area 

 equal to one square centimeter. 



By the same method we find the radius 

 of a circle whose area equals twenty 

 square centimeters, thus making each 

 of the ten spaces between circles 10 and 

 20 and bounded laterally by the ten radii, 

 equal to one square centimeter. "We 

 next construct a circle whose area equals 

 forty square centimeters and divide 

 each sector as far as circle 20, making 

 twenty equal areas between circles 20 

 and 40, each equal to one square centi- 

 meter. 



In like manner we construct circles 

 60, 100, and 140, dividing the sectors in 

 the zone lying between circles 60 and 140 

 to produce areas equal to one square cen- 

 timeter each. If a plate whose area is 

 greater than 140 square centimeters is 

 used, a circle whose area is 180 square 

 centimeters can be drawn and the radi- 

 ating lines extended out to the circle. 



The Petri dish can be centered upon 

 this apparatus by the circles and the 

 area read from the line its edges 

 approach. To facilitate the reading of 

 the area of the plate, the circles 80 and 

 120, whose areas are equal to 80 and 120 

 square centimeters respectively, were 

 drawn as dotted circles, thus making the 

 areas marked "a" and "b" equal to one- 

 half of a square centimeter. The colo- 

 nies in several areas can be counted, an 

 average taken, and the result multiplied 

 by the number of square centimeters in 

 each plate. 



To make an apparatus to use in the 

 laboratory there are various methods. 

 A very good way is to make a figure of 

 it upon tracing paper, from which a blue 

 print can be made. In the laboratory 

 of bacteriology at Cornell, the blue print 

 of the apparatus is framed. It is better 

 to have the surface of the paper a black, 

 which absorbs nearly all the rays of 

 light. A fine apparatus could also be 

 made by covering a plate of glass with 

 a uniform layer of wax, and with a 

 sharp instrument cut the figure in the 

 wax and subject it to hydro-fluoric 

 acid for a few minutes, which would 



etch the glass where exposed. Cleaning 

 off the wax and placing the glass plate 

 over black velvet the colonies of bac- 

 teria could easily be counted when the 

 plates are placed over the surface. 

 From the Pathological and Bacteriologi- 

 cal Laboratory, N. Y. State Veterinary 

 College, Cornell University, Ithaca, 

 New Tork. January 10, 1898. 



A New Colonometer. 



Julius Weiss. 



This colonometer has been devised for 

 counting colonies of bacteria on circular 

 plates. It is based on two geometrical 

 laws, viz., that the area of a circle is 

 equal to ttK- , and that the areas of 

 two circles are to each other as the 

 squares of their diameters. 



As will be seen from the figure, the 

 colonometer is made up of eight concen- 

 tric circles and ninety-two sector circles, 

 the last being definitely arranged within 

 the concentric circles. The first or cen- 

 ter xjircle is one centimeter in diameter; 

 the second concentric circle has a diame- 

 ter of three centimeters; the third has a 

 diameter of five centimeters; the fourth 

 of seven centimeters, the fifth of nine 

 centimeters, the sixth of eleven centimet- 

 ers, the seventh of thirteen centimeters, 

 the eighth of fifteen centimeters. This ar- 

 rangement of concentric circles has been 

 made to give wide choice in working 

 areas and to allow the use of Petri dishes 

 of different diameters. 



The concentric circles are divided into 

 equiangular sectors by eight common 

 radii. Circumferences 8, 7, 6, 5, 4, and 3 

 are again cut by eight lines, each of 

 these lines bisecting the parts of the 

 respective sectors that are included 

 between these circumferences. These 

 lines are therefore also radii, but which 

 have not been extended in the center. 

 The parts of the radii included between 

 the concentric circles are used as diame- 

 ters of the ninety-two sector circles; 

 each one of the sector circles has there- 

 fore a diameter of one centimeter, which 

 is also the diameter of the first or center 

 circle. 



The sector circles on two perpendicular 

 diameters have been divided into eight 

 equal sectors each, which is also the 

 number of sectors of the center circle; 

 the remaining sector circles are all 

 divided into quadrants. These divisions 

 enable the bacteriologist to do his count- 

 ing in very small areas when necessary. 



The counting is done as follows: An 

 average number of colonies is found in 

 one of the sector circles in a definite 

 working area, and this number is multi- 

 plied by the ratio of the area of the 

 sector circle to the area of the entire 



