444 SUMMAEY OF CURRENT RESEARCHES RELATING TO 



"The whole scale (fig. 66) is divided to 1/lOOths in., leaving the 

 field nearly unobstructed and free from the confusion effect of crowded 

 lines ; and these wide divisions may be used (taking advantage of the 

 middle lines in the subdivided spaces as a means of reading halves) 



Fig. 



with low powers where close work is not required. But every fifth 

 space is subdivided into ten, or 1/lOOOths in., and by using these 

 divisions for decimals, or these for units and the broad spaces for tens, 

 one may gain the precision of the finer scale with almost the facility 

 of the coarser. With a 1/10 in. objective the coarse spaces may be 

 made, with a moderate use of the draw-tube, to cover 1/10000 in., and 

 to read, with the assistance of one of the fine bands for tenths, 

 1/100000 in. A slight change of tube-length will give with equal 

 facility a reading by l/4ths of a micron (/x), or even 1/5 yx for easier 

 relations to decimal notation. 



Thus an average human blood-disc may reach from the line 

 marked 2 in the cut to about the 9th line in the fine band a, giving two 

 tens and nine units (29) by direct reading in 1/100000 in. Likewise 

 a disc of dog's blood may reach from line 2 to the 7th line of a, of beef's 

 blood from 2 to the 3rd line of a ; or of sheep's blood from 1 to the 9th 

 of a ; reading respectively 27, 23, and 19 one-hundred-thousandth of an 

 inch. Thus it would be easy to distinguish between all these except the 

 first two, and possible in that case, if we were certain as to the true 

 averages, and sure, which is more than doubtful, that the averages 

 themselves may not vary enough to obliterate the narrow margin 

 between them. 



Any one who can subdivide the smallest spaces to tenths, with the 

 eye, can of course read in millionths of an inch, or in fortieths /x, ; but 

 few persons are likely to go, at any advantage, beyond the record of 

 the finest lines. These appear wide enough apart to estimate in fourths 



