684 



Transactions of the Society. 



the greater amplification being as clear and sharp as with a power 

 of 300 diameters. For such use, the prismatic illuminating objec- 

 tives, manufactured by the Bausch and Lomb Optical Company, 

 are unequalled by anything the writer has ever used. In making 

 the measurements below recorded, low and medium powers only 

 were used, but always with a filar micrometer, the mean of from 

 five to ten readings of each end of every space being adopted. It 

 is the experience of the writer that when a series of measurements 

 is made, there is no advantage in using a very high power, the 

 results being sensibly the same with high, medium, and low 

 powers, having regard, of course, to the unit measured. 



It is not pretended that the measurements here recorded can 

 be depended upon to the last place of decimals in every instance. 

 No one knows better than the writer the difficulty inherent in such 

 work. The results recorded are, however, believed to be in the 

 main correct, and to reflect accurately the differences between the 

 various scales measured, all having been measured under sub- 

 stantially the same or similar conditions, except as otherwise 

 indicated. The tables give in every instance a short description 

 of the scales measured, the instruments used, the spaces measured 

 in terms of divisions of the filar micrometer, and the relative 

 corrections of each space in divisions of the micrometer and in 

 microns. The relative errors are determined by subtracting the 

 value of each space from the mean value of all the spaces measured. 

 A plus sign indicates that the space measured is shorter than this 

 mean, and a minus sign indicates that it is longer. The correction 

 for total length has been given in only a few instances, as the 

 object of this paper is not to standardise the individual scales, but 

 to determine their relative accuracy of graduation. 



The original observations, the results of which are here given, 

 are recorded in books K, L, and M. The references to " Record 

 Book " are to " Book Z," in which these results are tabulated. 



