II. MOLECULAR STRUCTURE. 351 



/. MICROMETERS. 



1797. An assortment of the finest Screw-Micrometers, of 



almost perfect accuracy, and a small instrument for observation. 



Hugo Schroder, Hamburg. 



This apparatus serves for the examination of screw micrometers, and 

 the micrometers which are exhibited are shown as examples of the great 

 accuracy and delicacy which can be attained by cutting the screws according 

 to the method invented by Hugo Schroder. A table showing the results of 

 the examination of one of the screws by Dr. Vogel is subjoined. 



1798. Position (Screw) Micrometer, constructed for the 

 refractor of the Royal Observatory at Berlin. 



Carl Bambcrg, Berlin. 



1799. Pour Micrometers, for Astronomical Telescopes. 



F. W. Breithaupt and Son, Cassel. 



Micrometer divisions on glass, for different kinds of astronomical observa- 

 tion. The one with .circular divisions was used by Prof. Sporer, of Anclain, 

 for the observation of the solar protuberances at Aden during the great solar 

 eclipse. 



1800. Electro-magnetic Registering Apparatus. 



M. Th. Edelmann, Physico- Mechanical Institute, Munich. 



ISOOa. Stereo-Micrometer. Apparatus used with binocular 

 telescopes, and measuring both angles and distances, for geodetical 

 and astronomical measurements. Both eyes being employed, one 

 measuring, the other observing, position and distance are thus 

 given simultaneously. With stereo-micrometrical photographs of 

 landscapes. Professor Carl Wenzel Zenger, Prague. 



II. INSTRUMENTS FOR DETERMINING THE MO- 

 LECULAR STRUCTURE OF THE HEAVENLY 

 BODIES. 



a. SPECTROSCOPES. 



1801. Spectroscope of Donati. 



Royal Institute of " Studii Superiori " at Florence. 



1802. Amateurs' Star Spectroscope. John Browning. 



This instrument will show the lines in the spectra of stars of the second 

 magnitude, when used with an object-glass only 3 in. in diameter, by de- 

 taching the cylindrical lens. The instrument may be used as a small direct 

 vision spectroscope. 



