81MPLE METHOD OF MICROMETRY. 233 



time, and then to jump over a space. This does not happen when 

 & point approaches a line ; or at least, to a far less amount, if at 

 all. 



Lastly, the present modes mostly require the assistance of a 

 draw-tube, in order to reduce or increase the value of the divisions 

 of the eye-piece micrometer to some definite relation to those of 

 the stage micrometer, so that it is necessary to enter in the table 

 which I have mentioned the length to which the tube has been 

 drawn out. Now, it is not every microscope that is provided with 

 such a convenience, although it is always present in the best and 

 most costly (adding, of course, to their cost). Besides, the use of 

 the draw-tube certainly impairs definition. 



Sir, I think that you and all my hearers will agree with me that 

 if any approach to equal precision can be attained in any science or 

 art, by the substitution of simpler methods or means, for those in 

 previous use, whereby also a saving of time and expense is effected 

 and facility of manipulation increased, a great benefit is gained, 

 since the value of that precision which proper and accurate mea- 

 surement, easilT/ effected, infers, cannot be overrated. In proof of 

 this, I need only refer to the labours of Mr. Whitworth in that 

 direction. It is on this plea that I submit my method 

 to you. It can, I think, scarcely be called an invention, 

 being rather an extension of the uses, or an adaptation to new uses 

 of a well known contrivance, called" Quekett's Indicator," as made 

 by Messrs. Beck, and it is this little instrument which I have 

 pressed into a new service, in which I hope to be able to show you 

 it behaves very well. This new service is micrometry of a direct 

 and positive kind, by the aid or use of the stage micrometer only. 

 Here, perhaps, I may be permitted to observe that I am glad that 

 I have not added to the number of insti'uments, but have merely 

 developed the uses of one of those already existing, for this has 

 been hitherto employed merely to point out some particular object, 

 or part of one in a field, to another observer. It consists of a fine 

 pointer, so fitted in the interior of an eye -piece, at the focus of the 

 eye-glass, as to be capable of being turned into or out of the field 

 by means of an external quadrant, properly milled, or, preferably, 

 by a small handle for the sake of greater power over it. Now, my 

 plan is to employ two of these indicators, either on opposite sides 

 of the eye-piece, or side by side. I prefer the former mode, as in 



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