ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



759 



•ends of these bars are small steel disks, by means of which they rest on 

 the vertical rods R ; by means of the spring S, R can be screwed up or 

 down, the point of M in the moist chamber falling if R is screwed up, 

 and conversely ; the screw on R has a very fine adjustment, and the 

 arm of the lever is about twice as long as the distance of P to the 

 point of the glass needle, so that very minute changes in position can 

 be made ; the pivot P is carried by a copper bar V, which is fixed in the 

 upright T. The glass needles have a stouter portion over the position 

 of the rod of about 3-4 mm. thick, opposite the pivot P about ^ mm., 

 and the fine ends are formed into points and loops, and vary in stout- 

 ness according to the nature of the organism to be isolated. The 

 Microscope being placed in position, the needles are laid in cement on 

 the holders, and are so arranged that the looped ends are directed 

 upwards, resting almost in the middle of the objective, but rather deeper 

 than the upper margin of the isolating chamber ; the side clefts are 

 ■closed, the cover-glass laid on top of the chamber, and the needles are 

 now pressed so deeply into the cement, that by moving the screw S the 

 ends can be made to rest on the under side of the cover-glass ; the 



Fig. 179. 



•ends of the needles should not be separated more than 300 fi, and should 

 not be exactly opposite each other. It is most important that the whole 

 •circumference of the loop should rest against the cover-slip. The cover- 

 slips recommended are 18 by 18 mm., thoroughly cleaned and lightly 

 spread with vaselin ; the moist chamber is a square glass frame, the side 

 walls being 2-3 mm. high and 5 mm. broad, making a capacity of about 

 14 by 14 mm. 



Before proceeding to the isolation of the cell, it is necessary to 

 ascertain how much of the material from which the isolation is to be 

 made must be added to a drop of f p.c. salt solution, so that there are 

 not too many cells at the margin of the drop ; the author gives details 

 of the method he employs to determine this suitable dilution. 



The suitable dilution being decided on and prepared in the specially 

 devised mixing chamber, the cover-slip on which the isolation is to be 

 made, is flamed and laid on the mixing chamber ; then with a sterilised 

 platinum needle are placed near to each other and equally distant from 

 the middle of the slip and rather to its left side, three drops of the 

 sterilised fluid in which the culture is to be made ; these are known as 

 the " culture drops " ; to the right of the middle and about 2 mm. dis- 



