ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 231 



ing, moreover, is within the Microscope, and therefore the time and in- 

 convenience usually spent in reading an external graduation will be saved. 

 The ocular has in its focal plane a scale divided on glass similar to ordi- 

 nary micrometric oculars. In contact with this scale, and also in the 

 ocular field, is a vernier divided into tenths, on the lower face of another 

 glass slip gliding on that of the fixed scale. A simple metallic slide, a 

 push-screw, and a back-spring suffice to move the vernier. The process 

 of measurement will be easily understood, and takes place in the field of 

 view. There are no special precautions to be taken, such as are usually 

 necessary, with regard to errors of screw, of springs, or of carriers, in the 

 case of external scales. Moreover, the measurement is made without 

 taking one's eye from the ocular, and it is possible, in case of need, to 

 dictate the readings to an assistant without the observer abandoning the 

 observation of a fugitive phenomenon. 



Watson and Sons' Holos Immersion Paraboloid.* — This appa- 

 ratus (fig. 2G) gives an intensely black background, with a brilliantly 

 illuminated object, with high-power objectives up 

 to 0'95 N.A., and is specially suited for showing 

 unstained living bacteria. The makers supply full 

 directions for the successful working of the appa- 

 ratus. 



Enumeration of Blood-corpuscles.j — R. Samut 

 advocates the use of the following simplified me- Fig. 26. 



thods for counting blood-corpuscles. 



The enumeration of the formed elements of the blood, although 

 admittedly of paramount importance in the diagnosis of disease, is not as 

 frequently carried out as its value would call for. This is undoubtedly 

 due to the fact that, in enumerating blood-corpnscles by means of Gowers's 

 or the Thoma-Zeiss haemocytometer, the chief difficulty encountered is 

 the necessity of counting the large number of corpuscles in each of the 

 sixteen small squares which make up one of the large squares, since at 

 least eight sets of sixteen small squares should be counted before a fairly 

 accurate result can be expected. Moreover, corpuscles often overlap the 

 lines which form the squares, and great care is required and time lost to 

 avoid counting them twice over. 



By means of the Blenden ocular " Ehrlich " these difficulties are 

 avoided. The construction of the ocular is as follows. An ordinary 

 No. 2 ocular is provided with a screen which cuts out a square from the 

 field of vision of the ocular. By means of the little knob (fig. 27) 

 this square can be narrowed, and by means of notches, which divide one 

 side of the square into four equal parts, the reduction may be effected 

 in exact proportion (fig. 28). 



Enumeration. — With this instrument enumeration of corpuscles is 

 done as follows. The drop of blood is obtained and diluted in the 

 Thoma-Zeiss pipette and blown out on to the Thoma-Zeiss ruled slide in 

 the usual way. When this has been placed on the Microscope, allow 

 5 minutes to elapse. Use a No. 9 Leitz objective and a Blenden ocular, 



* Watson and Sons' Special Catalogue, 1910. 

 t Lancet (1909) ii. pp. 1424 (2 figs.). 



