Exhibition Addresses. 175 



are copied by it with a literal fidelity, which renders the copy as higli in 

 authority as the original itself. In like manner venerable inscriptions, 

 hieroglyphics, and other nioniinients of antiquity of doubtful signifi- 

 cancy, situated in localities difficult of access, and, therefore, seen in 

 place only by the few, are reproduced with a fidelity equally unerring, 

 and placed in the hands of all tlie scholars of the world in their own 

 studies. The exact sciences have also recently foimd in photography 

 one of their most invaluable auxiliaries. By its aid the moon has 

 Ijeen mapped with an accuracy which leaves far behind even the 

 admirable work achieved with so immense- an expenditure of labor 

 by Madler and Baer. You are all of you familiar with the magnifi- 

 cent photographic moon maps by our accomplished townsmen, Mr. 

 Rutherford and Professor Draper, so that I need not dwell upon 

 them here. But Mr Rutherford has further apjjlied photography to 

 the mapping of tlie lines of the solar spectrum, furnishing the surest 

 means of determining their true relative positions and distances, and 

 thus contributing materially to the advancement of spectroscopic 

 chemistry. The same gentleman has made, also, a still more impor- 

 tant application of photography to astronomy. By receiving upon 

 plates of glass the images of whole groups of stars, and by applying 

 a micrometer of extraordinary power to the measurement of the 

 intervals between the several stars of the groups, he has provided a 

 means of fixing the true places of the stars in the heavens, greatly 

 superior to any heretofore employed ; while it enables the observer 

 to collect in a single favorable night material for determinations which 

 by previous methods would hardly be gathered in weeks or months. 

 A most competent judge, my friend Dr, Gould, of Cambridge, who 

 has practically tested the value of this method, pronounces it to be 

 the most important contribution to the instrumentalities of practical 

 astronomy which has been made since the invention of the transit 

 instrument, by Roemer. Finally, photography has been extensively 

 applied, during the great total solar eclipses of the.past few years, to 

 the purpose of fixing the images of those extraordinary luminous 

 appendages which are seen to border the great source of light while 

 its disk is completely covered by the moon. The brief duration of 

 the entire phenomenon of a total eclipse, tlie extreme variety of such 

 occurrences, and the unavoidable Excitement experienced by the 

 observer during the observation, are circumstance all unfavorable at 

 the moment to the critical study of the appearances presented. But 

 tlie numerous successful photographs which have been secured in 



