118 SUMMARY OP CURRENT RESEARCHES RELATING TO 



-^^ ; tbe lines of the band being well defined under all of tbem ; 

 and tbe eye-piece micrometer, Jackson's form, and a small spider-line 

 micrometer. Tbe former depends a good deal for its result upon an 

 estimation to tentbs of its graduations, and can bardly be susceptible 

 of tbe accuracy wbicb sboixld be attained witb a well-made ' wire 

 micrometer.' Tbe latter was therefore adopted and provided witb 

 additional draw-tubes, for use, either as an eye-piece in tbe usual 

 manner, or in the substage, giving an aerial image of tbe spider-lines 

 as proposed by Dr. Pigott.* This latter method, however, so far as 

 my own experience goes, is more ingenious than effective ; principally 

 because all vibration of tbe micrometer in that position is magnified 

 by tbe whole power of tbe Microscope. There is one advantage 

 possessed by Jackson's in tbe s^n-ing action, wbicb moves the whole 

 scale, and consequently its zero point, witb extreme nicety. In the 

 spider-line micrometer, one wire is generally fixed, and tbe only way 

 to bring a given point of an object under the Microscope to coincide 

 with that wire is by tbe screw action of the stage, which, witb a high 

 power, is far too sensitive and rapid. To obviate this difficulty, a 

 traversing movement to tbe extent of a fifth of an inch, controlled by 

 a screw of fine pitch, was added to tbe small micrometer between its 

 screw-plate and draw-tube. By this means any given line on the ruled 

 band, after being brought approximately into position witb the stage 

 movement, could be accurately bisected by tbe fixed wire of the micro- 

 meter. The objectives finally selected were a ^ for the measurement 

 of tbe principal subdivisions of 0*05 mm. each, and a -^-^ imm. for the 

 close spaces. These objectives gave the most convenient decimal values ; 

 tbe former by suitable adjustment of tbe draw-tube giving • 00025 mm. 

 as tbe equivalent of one division of the micrometer divided bead (50 

 divisions to one turn) ; and the latter "0001 mm. Both glasses were 

 by Beck, and their magnifying powers, witb the positive eye-piece 

 employed, were 950 and 2500 respectively. Of course the eye-piece 

 could be changed at jileasure, without altering the ratio of scale to 

 image. The fine movement of the Microscope employed is on its 

 main tube ; its action propels or withdraws the nose-piece, thus 

 possibly interfering with the value, as adjusted by tbe lengthening 

 draw-tube, of the micrometer scale in terms of a given unit. It 

 proved, however, by actual experiment, using a power of 1000 

 diameters, that an alteration of the fiftieth of an inch in the distance 

 from eye-piece to stage, made no perceptible change in tbe ratio 

 between the micrometer in the eye-piece and that on tbe stage, so any 

 supposed error in measurement from this cause may be dismissed as 

 visionary. All kinds of illumination were tried, the preference being 

 given to that described [in this Journal, I. (1881) p. 666], using 

 the concave mirror without condenser, at an obliquity of about 40°, 

 and a thin metal plate attached below the stage, at such an angle 

 that no rays from tbe lamp can reach the object, except by reflection 

 from tbe inclined mirror. With the light so directed, each line of 

 the band was evenly divided, longitudinally, into a dark half and a 

 light half, giving much facility for tbe exact superposition of a 

 * Mon. Micr. Journ., ix. p. 3. 



