[ 294 ] 



XXXV. On an Object-glass Spectral apparatus. 



By Sigmund Merz*. 



[With a Plate.] 



IN Gilbert's Annalen der Physik for 1823, Fraunhofer gives 

 a description of his observations upon the spectra of the 

 light of the fixed stars. He therein says, c< I have recently con- 

 structed a large instrument arranged solely for this purpose ; it 

 is provided with an object-glass the aperture of which is 4 inches 

 French measure ( = 4*26 English). The flint-glass prism of this 

 instrument has an angle of 37° 40', and is of the same diameter 

 as the object-glass." 



These are, without doubt, the earliest experiments on stellar 

 spectra. More than twenty years elapsed before further inves- 

 tigations on this subject were entered upon, first by Lamont, 

 then by Donati, Secchi, Jannsen, Huggins, Lockyer, and Z61U 

 ner, all of whom, however, had abandoned the method employed 

 by Fraunhofer, and commenced their observations by means of 

 an arrangement adapted to the eyepiece. 



Father Respighi in Rome has recently endeavoured to reintro- 

 duce the method of observation employed by Fraunhofer. His 

 first experiments gave such brilliant results that Father Secchi 

 wrote to me thereupon as follows : — "Professor Respighi relates 

 wonders to me about the prism you sent to him," and gave me 

 the commission to construct an object-glass spectral apparatus 

 for the refractor of the observatory of the Collegium Romanum. 



Now, as the apparatus in question has been for some time for- 

 warded to its destination, and as Father Secchi has given an ac- 

 count thereof in the Comptes Rendus of November 22, 1869, it 

 may not be considered out of place to make mention of it in this 

 periodical as well. 



Fig. 1, Plate III. represents the arrangement as mounted for 

 fitting on to the cell of the object-glass. Fig. 2 represents the 

 apparatus with the prism removed. Fig. 3 shows the prism 

 itselff. The refracting angle of this prism is 12°. It is worked 

 out of the purest and most colourless flint-glass, so that the loss 

 of light in traversing it may be taken to be =0. Its aperture 

 is 6 inches French measure ( = 6*39 English). The setting is 

 provided with the requisite arrangements for adjustment. 



Now, in spite of this prism reducing the effective aperture of 

 the 9-inch refractor at Rome ( = 9*59 English) by more than 

 half, the illumination far exceeds that of the refractor with 



* From Carl's Repertorium fur Experimental-Physik, vol. vi. p. 164. 

 Communicated by W. G. Lettsom, Esq. 



t Fig. 3 is of the size of tfee figure in the original plate. Figs. 1 and 2 

 are considerably reduced. 



