146 SUMMARY OP CURRENT RESEARCHES RELATING TO 



is of small magnifying power, and is rouglily focused by raising or de- 

 pressing the tube, while by turning a grooved ring in the middle of the 

 tube a fine-adjustment is obtained ; there is a fixed and a movable thread 

 with graduated circle. 



In using the instrument, the Microscope, fixed in the vertical position 

 by a stop on the axle S S which abuts on D, is first adjusted to the inner 

 ed«es of L and M successively by means of the micrometer-screw V; 

 (U and V have drums divided into 100 parts each, equivalent to a motion 

 of 0*005 mm.); this determines the distance between the edges and the 

 position of the experimental plate upon them. The screw s is then loosened 

 and the Microscope is rotated about the axle S S into the horizontal posi- 

 tion, where it is held by a second stop and counterbalanced by a weight 

 fixed on the lower end of T. It is then focused upon the upper surface of 

 the prism which is slightly curved ; the prism is raised until it just touches 

 the plate with its central point, and the interference rings are seen in the 

 field of view, when monochromatic light is reflected into the prism. If a 

 small space intervenes between the plate and the prism, then when the 

 plate is loaded this space is diminished and the interference rings travel 

 across the microscopic field, a motion through the breadth of one ring 

 beinc equivalent to a vertical displacement of half a wave-length ; in this 

 way the extent to which the plate is bent may be measured in fractions of a 

 wave-length. 



The apparatus atove described constitutes a complete micrometer for 

 the measurement of lengths and angles within the space of 4 sq. cm. 

 covered by the motions of the screws U and V, and may therefore be applied 

 to microscopical measurements for a variety of purposes. In this case 

 the object to be measured is placed upon an object stage, which rests upon 

 D above K and M, and is provided with a rotating glass plate mounted in 

 brass, and illuminated either with a lens from above, or from below with 

 the prism or a small mirror. In this form the instrument is especially 

 applicable to the measurement of Senarmont's or Eontgen's ellipse of heat- 

 conductibility, and to the examination of etched figures upon crystal faces ; 

 for the latter purpose it is particularly convenient when it is required 

 to measure the angle between the edge of an etched figure and an edge 

 of the crystal which is at some distance from the same, that is to say, when 

 the crystal is so large that a well-defined etched figure and the outline 

 of the crystal face are not visible together in the Microscope ; in such a 

 case the movable thread is adjusted to the edge of the etched figure ; the 

 Microscope is then shifted by means of the screws U V until the edge of the 

 crystal appears, when its direction may be determined by the movable 

 thread. 



Moginie's Travelling Microscope. — This (fig. 8) was designed by the 

 late Mr. W. Mogiuie, in order to provide an instrument which could be 

 very rapidly set up when travelling, and without the necessity of sepa- 

 rating it from its case. 



The limb supporting the socket for the body-tube and the stage is 

 attached by thumbscrews to the upper ends of two pairs of parallel bars, 

 the lower ends of which tarn on pivots fixed to the bottom of the box. 

 When the bars are depressed the limb, with the body-tube, stage, and 

 mirror, drops into the box. The loss of time in the operation of taking a 

 Microscope out of its box and replacing it again is thus avoided. 



At one end of the box are two flat rods or feet, turning on pivots and 

 allowing the box to be inclined, as shown in the fig. On the bottom arc 

 two similar feet which also turn on pivots, so as to extend horizontally on 



