and Laboratory Methods. 



1971 



Fig. 2. — Apparatus ready for use. 



on the top over the mirror, of a size to hold the 10(1 mm. Petri dish ; a rim is on 

 the circle to hold the dish in position. Underneath the circle a mirror, M, or a 

 black surface if de- 

 sirable, is placed at an 

 angle of 45 degrees. 

 A sliding post bearing 

 a jointed arm is set 

 into the block, A, to 

 hold the lens used in 

 counting. The dish 

 to be counted is set 

 upon the circle, the 

 cover is removed, and 

 the barrel is placed, 

 disk down, inside the 

 dish, the collar hold- 

 ing the barrel resting 

 upon the edge of the 

 dish. The barrel is lowered through the collar by means of the screw thread 

 until the ruled glass is close to the gelatine. The barrel is of sufficient length 

 that the ruled glass may be brought close to the gelatine in dishes of various 

 depths. By means of the jointed arm the lens is swung into place and may 

 be carried over the entire surface of the dish. 



An error in counting is easily made when the ruled glass rests upon the edge 

 of the Petri dish, because the eye in looking between the lines sees an area 

 greater than 1 sq. cm. Colonies almost underneath a line may very easily be 

 counted in each of two areas. By bringing the ruled glass close to the gelatine 

 the cause of such an error is eliminated. The lines on the disk are on the side 



toward the colonies so they may 

 approach very near. 



The barrel and collar are made 

 of such a size that they may be used 

 in dishes that vary a little in diam- 

 eter. If a dish with colonies is found 

 to be so small the disk will not go 

 inside, then the latter may be set on 

 top and the count made as with the 

 Wolfhuegel apparatus. Also, if at 

 any time it should be desirable to 

 make a count without removing the 

 cover of the dish, then the barrel 

 may be placed upon the table with 

 the glass disk uppermost, the dish 

 set upon it and the count made as 

 with the Lafar apparatus. 

 By sorting the Petri dishes already in the laboratory and by asking dealers 

 to supply dishes that do not vary much from the standard size, no difficulty will 

 be met in the use of the apparatus. 

 Drake University. L- S. Ross. 



Fig. 



3. — Barrel in collar, leaning upon edge of 

 Petri dish. 



