ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



657 



screw which fastens the lower system of a low-power objective to the barrel 



of the objective. Bend the points (1 and 2) down, so that they will meet 



and serve as a bristle clamp. 



Remove the lower system of the Fig. 183. 



objective, and put in the thin 



brass plate as in fig. 182 ; then 



draw a cat's whisker between 1 



and 2, and the finger will be 



ready for use as soon as the 



point of the whisker is in focus 



and in the centre of the field. 



A divided wire might be soldered on the ring in fig. 182, and it would 

 answer the same purpose (see fig. 183). 



Griffith's Substage Diaphragm-holder and Glass Diaphragms.* — Mr. 

 E. H. Griffith's holder is a metal disc a (fig. 184), which is to be fastened to 

 the substage fittings h, by means of the screw c, which allows it to be 

 turned in any position. An aperture of any desired diameter is made in 

 the holder a, and provided with a ledge for the support of diaphragms 



Fig. 184. 



which may be dropped into position when the holder is turned on one side, 

 as would be indicated in the fig. were the disc turned over. The slot at c 

 allows the diaphragm to be placed central with the objective on a decentered 

 stage. The screw-head at c should be of sufficient size to retain the holder 

 in any position it is placed. The pin d is to indicate a central position when 

 the holder is to be used on a well-centered stage. 



Thin metal discs with various apertures may be used for diaphragms, 

 but much cheaper ones may be made by placing common round cover- 

 glasses e/on the turntable, and with a brush quicSy covering all but the 

 desired aperture with asphalte or other pigment. In the place of diaphragms, 

 various coloured glasses for the modification of light may be used. 



Fleischl's Haemometer.f — This instrument, fig. 185, devised by Prof. 

 E. V. Fleischl for the estimation of haemoglobin in the blood, is based en 

 the colorimetric method ; that is, it compares the colour of red glass with 

 that of a solution of the blood, and from the thickness of the stratum of 

 the solution or of the glass when the tints are the same the amount of 

 colouring matter present in the blood is determined. Prof. Fleischl finds, 

 however, that although it is easy to prepare a plate of red glass which 

 has exactly the same colour as a certain thickness of a solution of blood, 

 yet if the thickness of the plate be increased n-fold it no longer has the 

 same depth of colour as a solution of the same blood concentrated »-fold, 



* Proc. Araer. Soc. Micr. 9th Ann. Meeting, 1886, pp. 150-1 (1 fig.), 

 t Med. Jahrb. K.K. Gesell. Aerzle Wieu, 1885, 20 pp, and 1 pi. 



