542 



On. the Prism Photometer 



prism combinations were discussed. Among these is to be 

 found the one which, as we now know, was invented by 

 Swan forty-one years ago. This form was in our opinion, 

 however, imperfect, and unsuited to technical requirements 

 for the following reasons : — Apart from the fact that the 

 balsam spot is not perfectly transparent, its edges do not 

 afford that extreme sharpness which can be attained by 

 ground surfaces, and they deteriorate by continually drying 

 up. In consequence of this the balsam-spot cube, as well as 

 some of the other prism-combinations perhaps superior to it, 

 has never been recommended by us, and we doubt whether 

 any considerable number have found their way into practical 

 use in technical industry. Should photometers with balsam 

 cubes, nevertheless, be now in use, they should of course be 

 called Swan's photometers. But let us not forget that only 

 with the most imperfect of all our described forms does the 

 cube of Swan coincide, and the coincidence only extends 

 to our " Gleichheit's " principle *. 



But we did not stop with the " Gleichbeits-Wurfel; " we 

 have also constructed a prism-combination which not only 

 satisfies all the conditions of the " Gleiehheits " principle, 

 but also allows the more sensitive "contrast-principle" +. first 



Fig-. 2. 



JP 



worked out by us, to be realized. The construction of this 

 prism combination with the help of a sand-blast is described 

 in our paper, " Photom. Untersuchungen I.," under cube 

 Xo. 5. Irs field has commonly the appearance indicated 

 in fig. 2. The different field- sections (I. -IV.) are separated 



* It may "be remarked here that the cube form had been already used 

 earlier by Fuchs for photometrical purposes, as mentioned in our 'paper. 

 As criterion he used the disappearance of the interference-bands, which 

 arise in the thin air-layer between the two prisms near the limit of total 

 reflexion, and which are complementary to each other in reflected and 

 refracted light. 



+ Fhotom. ITntersuchungeu, ii. : li Lichtmessung durch Schatzung 

 gleicher Helligkeiteunterschiede (Kontrastphotonieter)". Zeitschrift f. 

 ImtrhfJe, ix. pp. 461-465 (1889). 



