510 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
of three each were measured on Powell’s new scale : the variation from 
the mean was less than 1/200,000 in. Rogers’ is a very well ruled 
scale ; it is, however, difficult to observe, the lines being without pig- 
ment, and it is mounted dry. The lines under these circumstances 
present the usual black and white diffraction images. It is, on that 
account, very difficult to maintain an equable focus during measurement. 
In Rogers’ scale, the greatest error is in interval G, where it amounts to 
four divisions, or somewhat less than 1/100,000 in. Thirteen out of 
twenty intervals have practically an insensible error. One cannot 
speak with the same certainty with regard to this plate as to the others, 
because of the focal difficulty. Different readings gave discordant 
results ; therefore, in this case, more must be allowed for the ‘ constant 
of the observer and instrument.’ With regard to the l/10ths of a mm. 
on Powell’s scale, they were examined by a power of 600 diameters by a 
dry lens. The mean was 987 ; six intervals had no sensible error, but C 
and G had an error of three divisions, which is equivalent to 1/100,000 in. 
Rogers gave a very similar result. 
The error of the interval D, in the Zeiss scale, was 1/30,000 in. 
I next compared the length of the mm. on the three scales, that is 
Powell’s, Rogers’, and Zeiss’, with each other. I detected a slight but 
insensible difference of +1. All that now remains to be done, is to 
compare the inch and the mm. scale on Powell’s plate. By measurement, 
we found that 30 fx gave a screw value of 741 * 25 ; therefore, the value for 
•1 mm. would be 2470*8, and the value for 1/1000 in. 
*001 x 2470-8 627*59. 
*003937 
The value actually measured was, as we saw above, 628*0; here 
again there is no sensible discrepancy. In conclusion, I feel sure that 
such an accurately ruled micrometer, and one so clear to read, will prove 
extremely useful to microscopists at large. 
Before closing, I would like to bring to your notice a screw micro- 
meter made for me by Mr. Powell, which contains some slight modifica- 
tions from the usual forms, which practical experience has suggested to me. 
First, with regard to the lens portion, I have substituted a compen- 
sating positive for the old form of Huyghenian or Ramsden. This 
yields far better images when making measurements with apochro- 
matic and ordinary objectives. I have so arranged it that the compen- 
sating eye-lenses of different foci are interchangeable. In fact, 
no special lens is required, you use your ordinary working eye-piece, 
whatever that one may be. This is, of course, a great advantage : 
bacteria, for instance, require a high-power eye-piece micrometer, while 
such a power would be useless on an ordinary object. 
Therefore, the ability to regulate your eye-piece power to the object 
to be measured, will meet a long felt want. 
Next let me say that I entirely disapprove of having two movable 
threads; at the outset ‘the constant of the instrument’ would be 
doubled ; moreover, I am confident that a movable zero is a mistake. 
I have, therefore, considerably altered this portion of the instrument 
by making the screw portion, together with the fixed zero thread, 
movable in the other part, which might be aptly termed ‘ an eye-piece 
