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[ 541 ] 



LIV. iSo»<£ Observations in reply to the Paper of Prof . C. G 

 Knott " On Swan's Prism Photometer, etc" By 0. Lummei 

 and E. Brodhun *. 



UNDER the title "On Swan's Prism-Photometer, com- 

 monly called Lnmmer and Brodhun's Photometer/' 

 Prof. G. G. Knott calls attention, in the January number of 

 this periodical, to the fact that Swan had already, in 1859, 

 " invented, described, constructed, and used " the photometer 

 afterwards "re-invented" by us. The description given by 

 Swan of his photometer-cube at that time is also reprinted. 



'May we be permitted to offer some observations correcting 

 certain points in the above-named paper which seem to us to 

 be misleading? 



It is, first of all, certainly much to be regretted that this 

 publication has been delayed until now, although Prof. Knott 

 says the Swan photometer has been " familiar to all officially 

 connected with the Edinburgh University Physical Labor- 

 atory for some years past." 



Now, however, that Mr. Swan's precedence in the field is 

 brought to our attention after this long interval, we desire 

 to be among the first to acknowledge it. But the statement 

 that the photometer which we introduced into the technical 

 world is nothing more than a re-invention of Swan's photo- 

 meter seems equally unfair to us. 



Ffr.l. 



On the basis of theoretical considerations we formulated 

 the conditions necessary for a " Gleichheits-Photometer " of 

 the greatest possible sensitiveness. Following these in our 

 preliminary paper t, only that photometer-cube was described 

 which fulfilled the theoretical requirements of an ideal "grease- 

 spot " (fig. 1). This preliminary paper was followed, how- 

 ever, by our " Photometrische Untersuchungen " % t in which 

 the advantages and disadvantages of the various possible 



* Communicated by the Authors. 



f Zeitschriftf. Instrkde, ix. pp. 23-25 (1889). 



% Ibid, ix.* pp. 41-50(1889). 



