10 Scientific Proceedings, Royal Dublin Society. 



Lastly, when a slight loss of light is of no importance, a 

 glass plate with plane and parallel sides, the preparation o£ 

 which is a matter of no difficulty in the present state of 

 practical optics, may be substituted for the plane mirror P, 

 with the advantage of securing a large field, which may be 

 surveyed by a low power eyepiece, or by one of high power 

 so mounted as to be traversable over the whole field. 



Many other forms and modifications have occurred to us, some 

 of which may eventually be found practicable, but we deem it 

 unnecessary to crowd this paper with suggested forms which 

 have never been tried, and some of which will probably be 

 useless. 



The difficulties met with by Mr. Burnham, which originally 

 drew the attention of one of the authors (Mr. Grubb) to this 

 subject had reference mostly to adaptations of recording apparatus 

 to micrometers, as he felt the great inconvenience of rising from his 

 chair after making a bisection, probably in some most awkward 

 position, to read his micrometer head and circles with the aid of a 

 hand lamp. We have had some experience of recording apparatus ■ 

 attached to circle microscopes, and while we believe in their 

 feasibility, we question the desirability of such attachments to 

 delicate instruments.* We believe, however, that the same end 

 can be obtained by some simple adaptations to this new form of 

 micrometer. 



It does not take much imagination to conceive an optical 



* In some eases the recording apparatus consisted merely of an arrangement for pro- 

 ducing ink dots on an ivory head attached to micrometer head, and thus five or six 

 observations could be made, and all read off together. In the case of a special 

 micrometer, however, made for the Earl of Eosse, one of the authors (Mr. Grubb) 

 adapted a peculiar system which has not been elsewhere described. The micrometer 

 head terminated in a flat plate, represented in fig. 4 plate II., by the central circle a a. 

 Surrounding this, and in same plane, was a ring, 6 6 Z>, which formed part of a wheel 

 centered on the micrometer head, and by a simple contrivance caused to revolve once for 

 every ten revolutions of head itself. Surrounding this again was another ring, c c c, 

 stationary. The plates a a a, therefore, revolved with the screw directly, bbhaX ten times 

 this speed, and c c c was always stationary. In the centre was a small point, d, and 

 on outside ring two smaller points, e and / A similar needle point was fixed in some 

 part of each of the rings h bb and c c c, as at <j and h. A folding piece of brass, 

 covered with cardboard or indiarubber, was hinged in such a position that a piece of 

 paper could be conveniently applied to and stamped against the end of the 

 micrometer head. The stamping produced five holes, d e f g and h. A line drawn 

 through d e/gave the zero, the angular position of g gave the whole turns of micrometer 

 screw, and that of h, the angular position of the micrometer screw itself ; or, in other 

 words the reading of the micrometer head. 



In reading off these records the next day the paper was placed under a glass scale, 

 divided in one annul us, to read the whole turns and on the other the parts of a turn, the 

 point d giving the centre, and e and J" the zero of the scale. 



