562 RICHARDS— AUTOCOLLIMATING MOUNTING [April 20, 



pendicular to the direction of the Hght. In the autocoUimating 

 mounting none of these conditions are fulfilled. These disadvan- 

 tages are shared by all prism spectrographs. They are however less 

 with the grating, and, a matter of great importance, the amount of 

 the variations may be readily calculated and allowed for. 



The deviation of the spectrum from a normal one might seem to 

 be a serious objection, but as a matter of fact the deviation is much 

 less than might be thought. Thus on a plate of the usual size, the 

 maximum deviation from a normal scale is about one Angstrom, 

 and when using a comparison spectrum the maximum difference 

 between the corrections for two lines say ten Angstroms apart 

 would be about one twentieth of an Angstrom. Moreover the 

 deviation from the normal scale may be accurately allowed for, 

 being of the form 



Ji and ^2 being the distance of the line in question from the two 

 lines selected to establish the scale, and Aq the wave-length which 

 is returned through the slit. This correction is the same as that 

 necessary when using a plane grating in the autocoUimating position. 



The varying amount of dispersion is an objection in some classes 

 of work, especially where plates are taken in different regions of 

 the spectrum. The scale varies as sec (f>, and therefore is some- 

 what larger than that of the same order in the normal mounting, 

 especially when the angle is large. Moreover, in a given direction 

 the order of the spectrum is doubled and therefore the dispersion 

 is more than twice as great. This is an important property of the 

 autocoUimating mounting, since twice as many orders are avail- 

 able for observation. Thus with a 15,000 grating, four complete 

 spectra may be observed instead of only two. 



The inclination of the photographic plate will sometimes be a 

 more serious objection. Great care should be taken in its register. 

 This objection as somewhat weakened by the increase of the dis- 

 persion with the inclination, so that an error due to imperfect 

 register is proportional to sin <^ instead of tan </>. The error is 

 greatly reduced when using comparison spectra on the same plate. 



