320 Mr. Gassiot Description of a Rigid Spectroscope. [June 15, 



I. " Description of a Rigid Spectroscope, constructed to ascertain 

 whether the Position of the known and well-defined Lines of 

 a Spectrum is constant while the Coefficient of Terrestrial 

 Gravity under which the Observations are taken is made to 

 vary/' By J. P. GASSIOT, V.P.R.S. Received May 18, 1865. 



Shortly after my large spectroscope * had been removed to Kew Ob- 

 servatory, Mr. Stewart mentioned to me that he had had some con- 

 versation with Professor Tait of Edinburgh, as to the practicability of 

 having a spectroscope constructed so as to preclude all errors of observa- 

 tions arising from a displacement of the prisms or the shifting of any of 

 the fixed portions of the apparatus. 



The particular object Mr. Tait and Mr. Stewart had in view, was the de- 

 termining whether the positions of the known and well-defined lines of the 

 spectrum are constant While the coefficient of terrestrial gravity, under 

 which the observations are taken, is made to vary, Mr. Stewart consider- 

 ing that, provided an instrument could be constructed so rigid in all its 

 parts as to preclude all possibility of error, the observations might be 

 made in balloon ascents, varying from two to four miles. 



I consulted with Mr. Browning as to the practicability of constructing 

 the spectroscope. He considered such an instrument could be made, with 

 sufficient rigidity in all its parts, to examine with great accuracy any given 

 portion of the spectrum which might be selected, and for which the prisms 

 would have to be adjusted and fixed. I communicated with Mr. Coxwell 

 relative to the balloon ascents which would be required, and then deter- 

 mined on having the spectroscope constructed. 



On testing the alteration in the position of the lines arising from change 

 of temperature, it was soon ascertained that the difficulty of constructing 

 a truly rigid spectroscope was far greater than had been anticipated. 



By the description of the apparatus, it will be seen that 

 the prisms are arranged so as to bring the D-lines into the 

 centre of the field of view (fig. in margin), with a few of the 

 fainter lines on each side; a perpendicular fixed line, and 

 two cross moveable lines in the cobweb micrometer eye- n-iines as seen with 



. T i ^ the Rigid Spectro- 



piece, affording the means of measuring to IQ * QO of an scope. 

 inch, whatever alteration takes place in the position of the lines. 



The observations having been originally intended to be made in balloon 

 ascents, the construction of the spectroscope had necessarily to be con- 

 sidered in reference to some portable and easily manageable form, and it 

 was particularly desirable that its weight should be as low as possible. 

 These conditions were obtained by constructing and mounting it in a 

 T-shaped frame of gun-metal : in this manner the instrument was com- 

 pleted so as to weigh little more than 40 Ibs. ; but on carefully examining 

 the readings day by day in Mr. Browning's workshop, the errors arising 

 * Proceedings of Koyal Society, vol. xii. p. 536. 



