president's address. 195 



brought into focus on the same object, and the micrometer 

 screw is again made to butt on the top of the limb, its position 

 read off on the scales and noted as before. The co-efficient of 

 the instrument is the difference between these two readings, in 

 other words it is a micrometrical measurement of the difference 

 in position of the body with respect to the limb when a low- 

 angled dry and a wide-angled oil immersion objectives are 

 focussed on the same object by means of the coarse adjustment 

 only. For instance, let us suppose that the first reading was 

 27 - 38 and the second 39'24, the difference is 11*86, the required 

 co-efficient. The method of using the instrument is as follows : 

 — Suppose we wish to examine an object with an oil immersion 

 objective, the object is first focussed by the special low-angled 

 lens mentioned above, and when that has been precisely arrived 

 at the micrometer screw is turned until it butts against the 

 top of the limb, the scales are then read, the co-efficient ob- 

 tained above is then added to this reading, the micrometer 

 screw is now turned until it registers the sum of these read- 

 ings. The low-angled objective is then taken off, the oil 

 immersion objective placed on the nose-piece, and the body 

 racked down by the coarse adjustment until the micrometer 

 screw butts against the limb. The oil immersion objective will 

 now be in focus, provided that all the various steps have been 

 accurately performed ; the consequences would, however, be 

 disastrous if one made a slip in addition or an error in the read- 

 ing of one of the scales. Can anything be more ridiculous than 

 this setting and reading of a micrometer screw, the application 

 of a co-efficient, and the resetting of the micrometer screw 

 merely for the purpose of focussing an oil immersion objective, 

 an operation which a tyro in microscopy of a fortnight's stand- 

 ing could accomplish in a fourth of the time required for a single 

 reading of the micrometer. All this throws a flood of light — 

 first, on the state of microscopy on the Continent, and, secondly, 

 on the inefficiency of the continental stage, with its small hole 

 and spring clips. 



Before proceeding to the other part of the address two further 

 points may be considered. The first is, " How would you define 

 the size of a microscope ? " Obviously it is not its power, 

 neither the length or diameter of the body ; nor is it its weight 

 or the size of its stage. What then is it ? Practically the size 



