340 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



Micro-counting Slips.* — N. D. F. Pearce has lately had occasion 

 to make for the Lister Institute a number of micro-counting slips. 

 Those formerly in use are now unprocurable, being of German origm. 

 His slides (fig. 21) are simply an ordinary 3 by 1 slip on which are 

 cemented three pieces of thin glass of such a thickness that when a 

 drop of the preparation to be examined is placed on the central one, 

 and a cover-glass laid over all three, the film of liquid shall be • 1 mm. 

 thick. Using an eye-piece micrometer, with squares of known value, the 

 bacteria, blood corpuscles, &c., in so many fields are counted, and thus 

 the number present in any given volume of fluid can be calculated. 



The method of preparation requires care. The slides must be 

 chosen truly shaped and of uniform thickness. If this cannot be 



r— ^ 



Fig. 21. 



attained, a record must be kept of the calliper readings so as to know 

 the variations. The strips of glass must be a little too thick, and after 

 cementing on must be carefully and accurately ground down. It is 

 fairly, but not very, easy to get the final thickness • 1 mm. There are 

 many details as to the "'precautions necessary to secure success, and for 

 these the original paper should be consulted. 



Masonry Bases for the Installation of Microscopes and their 

 Accessories, including the Camera Lucida and the Microscope 

 Camera.t— N. A. Cobb advocates the advantages of installing Micro- 

 scopes, especially those intended for critical work, on heavy pillars 

 having foundations in the ground and passing upward through the 

 floors without contact. 



A similar method has long been in use in the best laboratories for 

 mounting instruments of precision. He describes in detail one such 

 installation which was carried out in cement and steel. Three girders, 

 two approximately eight inches in each transverse dimension, and 

 , between them a third smaller one, were imbedded vertically to the 

 depth of several feet in a l)lock of cement weighing many tons located 

 under the building. The middle short girder, extending 18 in. above 

 the floor, carries the Microscope and certain accessories connected with 

 illumination. The two tall, paired girders extend to within 1<S m. of 

 the ceiling, projecting upward into the room about 11 ft. The wooden 

 floor was laid tightly about the girders after they had been set in the 

 cement, and everything was then given a few months in which to settle 



* English Mechanic, cii. (1916) p. 573 (1 fig.). 



+ English Mechanic,' ciii. (1916) pp. 322-7 (5 figs.). Eeprinted from Trans. Amer. 

 Micr. Soc. 



