II. — On the Measurement of Grayson's Ten-hand Plate. 

 By A. A. C. Eliot Merlin. 



(Bead December 15, 1909.) 



Some years ago I undertook the laborious work of carefully 

 measuring all the spacings of my Grayson's ten-band stage micro- 

 meter. As such a task has probably not been undertaken by 

 many, I venture to communicate the results obtained in case they 

 may prove of interest to some Fellows of the Society. 



Two conditions appear to me of paramount importance in all. 

 delicate micrometrical work where the most accurate results are 

 aimed at : — 



1. An objective of comparatively very high initial magnifying 

 power should be employed. 



2. The micrometer eye-piece should be of moderate power, so 

 as not to unduly magnify errors which must always exist in even 

 the best screws. 



Under such circumstances, care being taken that the same 

 portion of the screw is utilised so far as possible, highly accurate 

 measurements may be effected. 



For spanning the rulings of the Grayson plate it was decided 

 to employ a nominal T ] g Powell dry objective. This lens is really 

 €'054 in. focus, has an initial magnifying power of a fraction over 

 185 on 10-in. tube, and its N.A. is 0*906. The optical index is 

 thus extremely low (4 "9), but, nevertheless, used under strictly 

 critical conditions in conjunction with the 6 eye-piece of Powell's 

 micrometer, the ruled lines appeared sharp, it being therefore 

 possible to adjust the "wires" very precisely against the diffrac- 

 tion edges of the rulings. Although the optical arrangement, 

 specified above, was selected at the time as the most suitable then 

 available for the purpose in view, it is by no means maintained 

 that the combination in question is the best possible, or the most 

 desirable, and since that time I usually employ an amplifier, 

 kindly specially computed by Mr. E. M. Nelson,* used in con- 

 junction with a Powell -^ oil-immersion objective of N.A. 1 • 27, 

 and thus augmenting its initial magnifying power about 2 • 5 times. 

 In order to obtain clear definition with the amplifier inserted 

 between the objective and ocular, it is necessary that the former 

 be provided with a correction collar, by means of which the 



* Sec this Journal, 1904, p. 396. 



