1892-93-] On Microscopical Measurement. 79 



better for us than they might have been, had not Mother Earth 

 passed through a great Ice Age. 



At this meeting Mr Hugh Fraser gave some interesting 

 "Notes on the Holly," illustrating his remarks by a large 

 number of specimens. Dr John H. Wilson also gave a lantern 

 lecture on Plant -hybridisation, dealing especially with some 

 remarkable hybrids raised by himself at St Andrews. 



IX.— ON MICROSCOPICAL MEASUREMENT. 



By WM. FORGAN, F.R.M.S. 



(Read April 28, 1893.) 



Three different and distinct things are embraced in this subject, 

 and may be taken in the following order : — 



Firstly, How to measure the size or diameter of any 

 microscopic object. To do this, it is necessary to have a 

 stage micrometer, ruled either with y^ths and -^^ ths of an 

 inch, or, as is done on the Continent, one millimetre ruled into 

 a hundred. A divided scale is also required to drop into the 

 eyepiece, but this scale does not require to be ruled in any 

 particular way, if only the divisions are equal. Then, when 

 the stage micrometer is on the stage and the divided scale in 

 the eyepiece, with a certain length of tube, if it is found that 

 tne TtJTT tn of an mcn on tne stage covers, say, five divisions 

 on the eyepiece, you know that, with that particular length of 

 tube and object-glass, each eyepiece division must represent 

 tlie 5txo tn of an incn - Then, removing the stage micrometer, 

 place an object on the stage, and if in its breadth or length 

 it fills one of the eyepiece divisions, it must measure the -g^th 

 of an inch. If it fills two eyepiece divisions, it must then be 

 tlie -2lh) tn of an mcn » and so on. It is well to find out the 

 value of these eyepiece divisions for each object-glass and 

 write them down on a card, to be placed in the microscope 



