700 Transactions of the Society. 



manner of movement is practically a sealed book ; what is wanted 

 is a more powerful Microscope. 



If these tubules through the silex are at the rate of 60,000 per 

 inch, and if their diameters are equal to the interspaces between 

 them, then the diameter of each tubule will be x^^q^q m - or 

 0-2116 micron. 



Now, assuming that the length of the tubule is four times its 

 diameter, its volume is so small that it would take 1381 years to 

 count the number that could be packed in a box 1 c.mm. in size, 

 counting 100 per minute and working 10 hours a day, 10 p.c. being 

 allowed for the interstices in packing cylinders in a cube. We are 

 apt to talk volubly about 1000 and 2000 diameters,*and what we 

 can see with those powers, without in the least realizing what we 

 are actually talking about. 



It is not so long ago that the Government, following the 

 example set by Geneva, instituted a laboratory at Kew for the 

 purpose of testing the rates of watches. According to their per- 

 formance, watches are divided into three classes — A, B and C — 

 and " especially good " is added to the certificate of a watch 

 obtaining a high percentage of marks in its class. We have not 

 yet got as far as this with our Microscopes, whatever we may 

 arrive at by-and-by ; in the meantime, I would not put any 

 Microscope into Class A unless it would demonstrate the sieve 

 membrane on the process of an Auliscus sculptus mounted in 

 balsam. This is an excellent test for several reasons. The dia- 

 tom is not rare ; a balsam mount is easily obtained, and is durable ; 

 the sieve does not vary much in strength ; the test cannot be 

 resolved by a narrow beam of oblique light in one azimuth, but it 

 requires a large axial cone to demonstrate it. It is sufficiently 

 delicate to test not only the objective, but also the eye-piece, the 

 condenser, the steadiness of the stand, the quality of the fine- 

 adjustment, and the keenness of the observer's sight ; but some more 

 difficult test than this would be necessary for an " especially good." 



