196 



PRESIDENT S ADDRESS. 



of a microscope depends on two things — first, the distance of 

 the optic axis from the limb ; and, secondly, its distance from 

 the table when the microscope is placed in a horizontal position. 

 Although a simple it is an important and suggestive question, 

 which has not received much attention hitherto. 



The second point has reference to the conversion of metric 

 into English linear measures, and vice versa. 



The best determination of the length of the metre by Colonel 

 Clarke in 1866 made it equal to 39-37043196 inches, but as 

 few people can remember a numerical co-efficient for reduction, 

 a well-known rough and ready substitute, sufficiently near 

 for practical purposes, and with a memory catch in it, is used in- 

 stead of the correct figure. It is as follows : One metre is 

 equal to three feet, three inches, and one-third. If you work 

 this out you will find that it is too short by five feet in a mile. 

 Now could not a much nearer equivalent be found which, like 

 the precpding, possesses some sort of memory catch ? After a 

 little search I succeeded in finding one that will probably meet 

 most requirements. The number to be remembered is six and 

 a half inches. This should not be difficult to a microscopist, 

 because it is the length of the continental tube. Now all that 

 is necessary to convert this into metrical measure is to write a 

 one in front and a one behind it. Therefore 65 will become 

 1651, that is six and a half inches is equal to one hundred and 

 sixty-five decimal one millemetres. The error in this co- 

 efficient is only five and a half inches in ten miles ; it is, there- 

 fore, about 110 times more accurate than the other memory 

 co-efficient. 



All microscopists who have to reduce metrical measures, or 

 convert English measures, or to work with micrometers, should 

 most certainly invest in a slide rule. With a slide rule any 

 reduction becomes supremely easy ; all that is required in the 

 present case is to set 1651 on the slide in a line with 65 on the 

 rule ; when that is done any conversion of metric into English, 

 or English into metric measure, is performed by inspection 

 without any further movement of the slide. For example, 

 when the slide is so set 3 on the slide will be in a line with 118 

 on the rule, therefore 3 mm, equals "118 inch, or 30 mm. 118 

 inch; again, 315 on the rule is in a line with 8 on the slide, 

 therefore 3*15 inch is equal to 80 mm., or '315 to 8 mm., and 



