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PROCEEDINGS OF THE AMERICAN ACADEMY 



The geueral principle of tltis form of construction, and tlie methods 

 of comparison era|)loyed, will more clearly appear from the drawings 

 and description of the Universal Comparator which follow. 



The Rogers-Bond Universal Comparator. 



This comparator is the outgrowth of experience with the apparatus 

 first constructed. In 1880, through the kind offices of Professor J. E. 

 Denton, of Stevens Institute, Mr. Geoi-ge M. Bond, then a member of 

 the Senior Class, came to Cambridge and made the sketches, from 

 which he afterwards executed the full drawings. Mr. Bond was 

 intrusted with the entire arrangement of all details, and to his judi- 

 cious application of sound mechanical principles in the form of con- 

 struction is due in a large measure the successful working of the 

 apparatus. 



Soon after this, Messrs. Pratt and Whitney, of Hartford, Conn., un- 

 dertook the construction of two machines from these specifications, one 

 of which was to be used as an auxiliary in the investigation and con- 

 struction of the system of gauges which they have established. These 

 comparators were completed in May, 1881 ; but it was not until the 

 September following that the machine constructed for the writer was 

 completely mounted at Cambridge. 



In de!<igning this apparatus, the following requirements were kept 

 steadily in view : — 



(1.) The complete separation of the standards to be compared 

 from the frame-work to which the microscopes are attached. 



(2.) The best method of securing an invariable reference plane, 



