458 SUMMARY OF CURRENT RESEARCHES RELATING TO 



For standard end-measurements, wliere the difficulty is to determine 

 the precise points of contact, the object is placed in a double V-carrier, one 

 end touching a fixed electrical contact-point, the other end is then presented 

 towards a travelling contact-point, actuated by the micrometer-screw, and 

 the contact is shown by the deflection of a delicate galvanometer needle to 

 an estimated accuracy of about 1/1,000,000 in. For registering tempera- 

 tures, a thermometer is attached to the micrometer frame. 



In practice the stage-plate on which the object is placed is first levelled 

 by means of a spirit-level, then the tripod of the micrometer-frame is 

 adjusted in the V-slots on the base-plate and accurately levelled, for which 

 purpose spirit-levels are applied to the frame at right angles. 



For high-power work the Microscope is furnished with Mr. Hilger's 

 tangent-screw fine-adjustment, in which the motion is unusually slow, and 

 which is described infra, p. 461. 



Watson - Draper Microscope. — This Microscope (Plate IX.) made by 

 Messrs. Watson & Sons, after the designs of Mr. E. T. Draper, is an 

 elaboration of the instrument suggested by Mr. E. Crossley.* The follow- 

 ing description is furnished by Messrs. Watson : — 



The idea in arranging it is, that when the object is on the stage, either 

 it may be made to rotate in any direction, horizontal or vertical, round a 

 fixed beam of light, without the light ever leaving the object, or the stage 

 may be kept fixed while the light is revolving round it in any direction, 

 horizontal or vertical ; always, how^ever, remaining upon the object. Of 

 course to do this exactly it is absolutely necessary that the object should 

 be precisely in the centre of all the circles in which the various parts of 

 the instrument are revolving, and to enable this to be done with the utmost 

 precision, there is an adjustment to the stage by means of a micrometer- 

 thread screw below, to raise or lower it according to the requirements of 

 different thicknesses of objects. 



The body is mounted on an extremely solid pillar carrying a quadrant 

 of a circle, and in this it may be j)laced in any position from the horizontal 

 to the vertical, and as the stage is connected and moves with the body, and 

 as this arc of a circle is struck from a radius, the centre of which would 

 be the object on the stage, it follows that when light is thrown from 

 directly underneath the object, by inclining the Microscope through this 

 arc and without touching the mirror, the light becomes more and more 

 oblique, till it arrives at that point where it is impossible for it to enter 

 the objective. Again, the stage being a concentric rotating one, allows the 

 object to be moved horizontally round the same fixed light. The two 

 motions therefore are used when it is desired to place the object in any 

 position with regard to a ray of light. 



For those objects, however, which could not be conveniently moved, 

 there is another arrangement for keeping the object stationary, while the 

 light is thrown upon it from any desired angle. This is done by using 

 Mr. Crossley 's arrangement of a train of prisms transmitting the light on 

 to the mirror and rotating on an axis in the same plane with the object on 

 the stage. The prisms have also an additional movement which Mr. 

 Crossley's arrangement has not, viz. the pillar supporting them is fixed 

 upon a horizontal rotating base-plate so that by the movement of the base- 

 plate, combined with the swinging motion of the prisms, the light may be 

 thrown through them upon the object from any direction, horizontal or 

 vertical. A lamp is fixed permanently to the pillar carrying the prisms, 

 which moves with it in whatever direction it is placed. There is also a 



* See this Journal, 18S1, p. 053. 



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