ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 519 



It is obvious that witli this device the power employed is always 

 the same, when once adjusted, and enlargement up to 5000 diameters 

 may be obtained. The micrometer eye-piece, where the body is 

 moved by the fine adjustment,]is also practically unchanging in power, 

 but cannot easily afford the same amount of magnification, unless with 

 unusually high-power objectives whose short working distance usually 

 precludes their use with tube lengths sufficient to give so great 

 amplification. 



A very convenient method of using the focusing stage in micro- 

 metry is to so adjust the screen that '001 in. of the stage micrometer 

 exactly equals 1 in. of the paper scales used by architects and 

 divided into hundredths of inches ; by pasting one of these scales 

 across the screen and bringing the micrometer lines (of 0*001 in.) to 

 coincide with the inch lines of the scales, and clamping the screen in 

 that position, a scale upon the screen is obtained reading to rows^^^'' 

 which is far finer than can ordinarily be utilized, although by sunlight 

 the striae of some diatoms, such as F. saxonica and A. pellucida, will 

 puzzle the eyesight in attempting to count their striation by means 

 of the scale. 



An incidental feature of this focusing stage is that it will not allow 

 the slide or cover to be broken in focusing, and is therefore a safety 

 stage as well. 



In making measurements by this method the same spaces of the 

 scale should be used for every measurement, and, preferably, the 

 central ones, thus removing any question as to the variation of power 

 or aberrations in the extreme edges of the field. Thus, if the objects 

 measured are about 1 J or 2 divisions of the scale, and two are in the 

 field at once, do not read the dimensions and record them as they 

 stand, but bring first one of the objects to the central line and read 

 from that, and note the measurement ; then bring the other object to 

 the same side of the central line, read and record as before ; both are 

 then measured by the same part of the scale to the extent of the 

 smaller. 



Logan's Life-Slide.* — Mr. J. H. Logan's slide (fig. 103) consists 

 of a glass slip of the usual size, but 1/4 in. thick. An annular channel 



Fig. 103. 



IJ \J- — 



as deep as the thickness of the slide allows is ground out for an air- 

 space, and outside of this a much narrower and quite shallow channel 

 is cut. This last is for holding beeswax or wax and oil, to cement 

 down the cover and prevent evaporation of the enclosed fluid. A drop 

 of water placed in the centre of the slide and flattened down to a 

 stratum as thin as the objects under examination will permit, is in 

 a very favourable condition for examination. Infusoria, thus confined, 



* Proc. Amcr. Soc, Blicr., 8th Auu. Meeting, 1885, pp. 110-1 (1 fig.). 



