216 



NOTES AND CORRESPONDENCE. 



Micrometers. — Few subjects have been more frequently dis- 

 cussed among microscopists than that of the relative value of 

 the various micrometers in common use ; and^ some time ago, 

 I remember a stricture upon Mr. Quekett for having in his 

 ' Practical Treatise/ &c., said of Ramsden's micrometer (the 

 double cobweb) that, " as its accuracy depends entirely on 

 that of the glass micrometer used in finding the value of its 

 divisions, the measurements made by it are by no means so 

 delicate as they appear to be." 



There is truth in the above : nevertheless it would have 

 been better if, instead of the word, " delicate," he had used 

 satisfactory. The fact is, all micrometers on the principl? of 

 Ramsden, Jackson, &c., must remain unsatisfactory if we 

 have no ready means at hand of proving the accuracy of the 

 glass micrometer itself; for though an exhibitor may tell his 

 friend " the divisions on this glass micrometer are exactly the 

 one hundredth of an inch," nothing is more likely than that 

 his friend will say, " But how do you know that ?" To which 

 we may reply, " I had the glass micrometer from a first-rate 

 London optician, who, I am confident, would not supply a 

 defective article," &c. But to this many would answer, 

 " Ah ! that does not satisfy me. I should like to see some- 

 thing like proof, and not mere statement," &c. 



This is but reasonable ; and therefore it is my opinion that 

 every user of our favorite instrument, to whom expense is 

 not an object, ought to have by him a means of proving the 

 accuracy of his graduated glasses ; and for this purpose I 

 believe nothing is better than the mechanical stage micro- 

 meter I alluded to in the note, p. 270, in your Journal for 

 last October. It is not only a good micrometer in itself, but 

 it also enables us to prove the accuracy of all others. So that 

 though I have all the raicrometrical apparatus I have ever 

 heard of — Ramsden^ s, Jackson's, Nobert's, &c. — and a variety 

 of glass micrometers by various hands, yet I consider the 



