144 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



in 1851, having four faces, with angles of 57°, 150°, 48"", and 105°, tho 

 rays being twice totally reflected. 



Berger's Microscope for fixing Spider's Threads.* — Mr. C. L. Berger 

 describes the following Microscope for the adjustment of distance-threads 

 in telescopes for theodolites, &c. To fix the threads in the grooves is used 

 a small apparatus h (fig. 6), which stands upon a rotating plate a ; b can 

 be both rotated about the pin c, and moved backwards and forwards, and 



Fig. 6. 



clamped by the screw h. The apparatus can also be moved with the 

 stand e to different parts of the plate, so that the diaphragm (for the 

 telescope) need not be moved. The latter is held by a spring on the little 

 stage d in the centre of the table a, and there is a mirror under d. 

 "With this apparatus he has been able to adjust the distance-threads for use 

 with normal levelling stafi"s to within • 001 of their true position, which 

 corresponds to an error of 0*1 foot at a distance of 100 feet. This error, 

 especially with long distances, lies within the limits of the accuracy which 

 can be attained with distance-threads in general, and may in most cases 

 be neglected. By using a micrometer-screw with the Microscope, as is 

 done with dividing machines, the threads may be still more accurately 

 adjusted before they are fixed to the diaphragm, and the error still further 

 reduced. 



Koch's'' Microscope for determining Coefficients of Elasticity.f— The 

 apparatus originally devised by Dr. K. R. Koch for his experiments on the 

 elasticity of crystals, is now made in an improved form by Breithaupt and 

 Son, of Kassel, and is shown one-fourth natural size in fig. 7. 



A solid stand A, of lacquered iron, supported on three levelling screws a, 

 carries the steel bar D on which is fixed the steel anvil M ; the upper sur- 

 faces of M and of a similar anvil L are slightly bevelled upwards, so that 

 the plate or bar to be examined, when placed upon M and L, rests upon 

 their inner edges alone as two linear supports; L is, however, not fixed, 

 but is suspended on a knife-edge forming part of K, parallel to the length 



* Zeiischr. f. Instnimcntenk., vi. (1886) p. 276 (1 fig.)- 



t Groth, P., ' Fhysikalische Krystallogiaphie,' 2nd ed., 1885, pp. 660-6, 3 figs. 



