794 SUMMARY OF CURRENT RESEARCHES RELATING TO 



position of an object seen througli the Microscope will be tbe same as 

 when they are used without a Microscope, i. e. so long as the plates are 

 in one plane the image is unchanged in its position, but as soon as 

 the plates cross at an angle it will be separated into two images of 

 equal size, which are displaced in opposite directions. By turning 

 the plates through a sufficient angle the displacement can be so 

 arranged that the margins of the two images which are turned to each 

 other shall coincide, and a compound image is formed which, in the 

 direction of the displacement is twice as large as the original 

 one. For the same eye objective and eye-piece and for a constant 

 distance of both from the axis of the plates, the angle of inclination 

 to be given to the plates, in order to double the image, bears a fixed 

 relation to the size of the object and may therefore be used to 

 measure it. 



If a table is prepared showing the values in mm. of the angles of 

 inclination of the plates, it is only necessary in measuring an object 

 to turn the plates until the image is doubled and ascertain the angle 

 between them, and the table will then give the dimensions. 



In fig. 123 the square box between the eye-piece and objective 

 holds the Helmholtz plates which are rotated by the outer milled 

 head, the angles of inclination being read off on the large graduated 

 drums on each side. 



It is claimed that by the use of this instrument those errors are 

 avoided which arise in the use of the eye-piece micrometer if the 

 image of the object does not exactly fall in the plane of the 

 micrometer divisions. The angles can moreover be read with greater 

 precision than the micrometer divisions. 



Baumann's Callipers with Movable Microscope and Fixed 

 Micrometer.* — T. Baumann's instrument (fig. 124), in which the 

 Microscope is movable and has a fixed micrometer in the eye-piece, is 

 not intended for such minute measurements as the preceding, but was 

 devised for cases for which a vernier is not sufficiently exact, while a 

 screw micrometer is too fine or not sufficiently rapid. It will read 

 to • 04 mm. In a base plate A A, 200 mm. long, a central groove is 

 cut, along which moves the cylinder a. The upper edges of the 

 groove are bevelled off by a cylinder of the same diameter as a. The 

 cylinder moves freely along these without attachment of any kind, to 

 avoid errors of tension, &c. To one end of the cylinder is attached a 

 glass plate C, another glass plate B being fixed exactly parallel at the 

 end of A, the two plates forming the jaws of the callipers. The 

 cylinder is moved by the ivory handle at h. A plate u u is attached 

 to the former on one side, to which plate are fastened the two supports 

 g which carry the socket of a compound Microscope I o (78 mm. high 

 and magnifying 50-60 times). The supports g rest on the base plate. 

 The socket is divided and the two halves are clamped by the milled 

 head m. The inside of the socket has a worm so that by turning the 

 ring k the Microscope is moved up or down for focusing. 



The edge of the base plate is divided on silver for 150 mm. into 



* Zeitschr. f. Instrumentenk., iv. (1884) pp. 149-52 (2 figs.). 



