1080 RECORD OF CURRENT RESEARCHES RELATING TO 



the top of the ring when the instrument is upright. The stand is 

 12i inches in height, and the body and draw-tube are each 5 inches 

 in length. The fine adjustment moves the whole body-tube, and there 

 is the broad-gauge screw for high-angle low-power objectives, in 

 which fits an adapter with the regular Society screw. 



Other details are shown in the figure, which is about two-fifths the 

 actual size of the instrument. 



Standards of Length— Uluminatioii for Opaque Objects.* — 

 Professor "W. A. Rogers has T)ublished an exhaustive paper on those 

 standards of length which are in actual use, and which have the 

 authority and sanction of either national or international law. Much 

 of the paper is beyond our scoj^e ; but the author refers to two points 

 bearing upon the use of the Microscope in verifying standards. 



With regard to the magnifying power of the Microscope employed, 

 which is best adapted to secure the greatest absolute accuracy in 

 measurements, the result of the author's experience on the subject is 

 favourable to high powers. With a proper illumination, and with 

 lines having smooth edges, a power of 900 can be used with great 

 ease, even in the comparison of two metres upon a longitudinal com- 

 parator. In all the earlier comparisons Microscopes of very low power 

 were employed, varying from 10 to 60 diameters, and the International 

 Commission have decided upon the low power of 40 to 50. M. Tresca, 

 of the French section, however, is a firm believer in high powers, and 

 prefers one of about 400. 



On the best method of illumination for opaque objects, Professor 

 Eogers says — " I cannot better illustrate the necessity for a proper 

 illumination in making exact measurements than by saying that I have 

 been obliged to reject a series of observations, extending over a period 

 of four months, for the simple reason that I finally discovered that, 

 during all this time, I have never once seen the actual lines ruled, but 

 only their image. I used a parabolic reflector, giving a beautiful 

 white line on a black background. The lines were traced upon a steel 

 surface, nickel-plated, their width being about one ten-thousandth of 

 an inch. Investigation showed that the positions of the lines could 

 be changed by an amount more than half their width, by shifting the 

 position of the parabolic reflector. 



The method of illuminaticm employed by Baily and Sheepshanks 

 seems to me radically defective. With the Microscopes used by Sheep- 

 shanks I found myself unable to separate lines ruled on a polished 

 steel plate, though sepaiated by an interval of only one-thousandth of 

 a centimetre. As already stated, I have used with great satisfaction 

 the form of illumination described by Mr. Tolles in the ' Annual of 

 Scientific Discovery' for 1866-67. f It is sufficient to say here, that, 

 as none of the light is lost by the reflection, it is easy to get all, and 

 even more than is needed. Diflused daylight falling upon the j)lane 

 face of the prism inserted between the two front lenses afibrds an 

 abundance of Kght for the most delicate tracings. With a 1-inch objec- 



* 'Proc. Am. Acad. Arts and Sci.,* xv. (1880) pp. 273-312. 

 t See this Journal, ante, p. 7.54. 



