6 Mr. J. P. Gassiot on Observations [June 20, 



In conclusion, it may be desirable to describe in a few words the 

 method by which a sextant may be verified by means of this apparatus. 



Let us suppose the collimators to be accurately and quite immoveably 

 fixed, and the angular distances between them to be accurately deter- 

 mined. Also let the distance between the two lenses a and h of any 

 collimator be such that the collimator E may be seen through a at the 

 horizon-glass, and the collimator Gr through h at the index-glass of an 

 ordinary sextant placed on the table B. 



In order to test the index-error of a sextant, the vertical line of a 

 collimator is made to bisect the cross lines belonging to the same 

 collimator in the field of view of the telescope of this sextant. If the 

 sextant is accurate, it should read zero, since these two lines are infi- 

 nitely distant and in the same direction. 



The sextant is next placed with its horizon-glass receiving the rays 

 from the vertical line collimator (Gr^ being covered), and its index- 

 glass receiving the rays from the cross line collimator Gr-, and the tele- 

 scope arm is moved until bisects Gl-g in the field of view ; if the in- 

 strument is correct, the reading ought to be (by Table III.) 15° 0' 10" "4. 

 By pursuing this method it is evident from Table III. that the error of 

 graduation of the sextant may be determined at every 15° of its arc. 



In conclusion, it ought to be mentioned that perhaps no artifical light 

 easily obtainable is sufiiciently powerful to allow of the darkest glasses 

 of a sextant being examined, and that for this purpose we may ultimately 

 have to resort to other means. 



II. " On the Observations made with a Rigid Spectroscope^ by 

 Captain Mayne and Mr. Connor, 2nd Master of H.M.S. 

 ' Nassau/ on a voyage to the Straits of Magellan.^^ By J. P. 

 Gassiot, F.E.S. Received May 25 and June 3, 1867. 



In a communication I made to the Royal Society on the 18th of May 

 1865 (Proceedings, vol. xiv. page 320), I described the rigid spectroscope 

 which, at the suggestion of Mr. Balfour Stewart (in connection with a 

 plan jointly conceived by Prof. Tait and himself), I had had constructed, 

 the object sought being to determine by observation whether the index 

 of refraction does not vary with the coefficient of terrestrial gravity, for 

 which purpose it was thought desirable that the observation should be 

 entrusted to some officer on board one of H,M. ships visiting various 

 latitudes on both sides of the equator. 



Through the kindness of Captain Richards, Hydrographer to the 

 Admiralty, I obtained an introduction to Captain Mayne, of H.M. 

 Ship ' Nassau,' at that time (August 1866) fitting out at Woolwich, 

 preparatory to making a survey of the Straits of Magellan, and by 

 appointment with Captain Mayne I visited the 'Nassau' in company 

 with Captain Richards. 



