30G .^lEMOUAND.A. 



like a 1-ltli to be a l-Uh, and a ISt/t a l-8th, or else to be 

 told what they really are. 



Wenham's Adjustment for Covering Glass. — Some time 

 near the end of 1858 I resolved to treat myself to a l-12th. 

 I had read in the Journal Mr. Shadbolt's remarks upon Mr. 

 Wenham's method of eflecting this adjustment, and I deter- 

 mined my l-12th should have it. Mr. Wenham, to whom I 

 am an utter stranger, was kind enough not only to explain 

 to Messrs. Powell aiid Leland his method of applying it, but 

 also to examine the objective when finished. This adjustment 

 is such a luxury compared with the old method, that it will 

 perhaps justify the inquiry, why has it not been adopted 

 by our makers ? It is just as easy to make, I think easier, 

 and I do not think any one who has used it for a week could 

 by any possibility be satisfied with the old method ; I may 

 therefore ask again, why has it not been adopted ? 



Micrometry. — A good deal was written some time ago 

 about the best form of micrometer. I do not wish to provoke 

 a fresh discussion of this subject, and will therefore only say 

 that no man who has conscientiously endeavoured to meke 

 exact measurements of minute objects can by possibility 

 be satisfied with anything less refined than the cabinet micro- 

 meter. With a fixed scale like the glass micrometer there 

 never can be anything but guess-work. Who, for instance, 

 would undertake to measure the blood-corpuscles with such 

 imperfect means ? No one, I am certain, who was qualified 

 for the task. 



But in using a cabinet micrometer with high powers and 

 the ordinary stage movements, it is very difficult to make 

 exact contact with the fixed filament ; and although it may be 

 done in time, it is an exceedingly great trial of patience when 

 it has to be repeated, and, what is worse, a great loss of time. 



To remedy this inconvenience I suggested to Mr. Thomas 

 Ross an alteration of the object-plate, upon wdiicli, as a 

 matter of course, with his better mechanical knowledge, that 

 gentleman improved, and I have found it so exceedingly con- 

 venient that I feel bound to recommend it to those micro- 

 scopists who desire to make accui'ate measurements withthe 

 least expenditure of time. It would not probably cost much 

 to alter a stage or to apply it to a new instrument, but in 

 consequence of my absence from England I was obliged to 

 have a new object- plate made. 



There are people who say that stage movements of any 

 kind are a superfluity, or something worse. Of course those 

 nimble-fingered and clever manipulators will not require Mr. 

 Ross's help, but the clumsy fellows who, like me, still wish 

 to be accurate, perhaps will. I have ascertained that when 



