934 
MR. A E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
of Berlin, is used as refractometer. This instrument, and the supporting stand which 
raises it so that the optical tubes are brought to the level of the eye, are placed in 
front of the diffusing tube of the monochromatic light apparatus, just as were the 
axial angle goniometer and its stand. The form of stand shown in the illustration is 
particularly convenient, both for ordinary goniometric and spectrometric observations. 
The lower and broader base of polished mahogany, covered beneath with cloth, so as 
to be easily moved over the polished table, serves as support for a protective glass 
shade when the instrument is not in use. Upon this rests a second base of smaller 
surface but of about the same height, about four inches, and of similar polished 
mahogany; it may with advantage carry a drawer in which the accessories of the 
instrument may be kept, and is covered underneath with cloth so that it can be 
moved easily over the larger base. The levelling screws of the goniometer rest in toe 
plates, also covered underneath with cloth, placed upon the surface of the smaller 
base. The weight of the instrument is ample to prevent movement during the 
observations, while this mode of mounting enables the goniometer to be placed in any 
convenient position upon the upper base, and the latter as well as the lower base to 
be independently arranged in the most convenient positions for the work in hand. 
The illumination tube supplied with the goniometer, to be placed in front of the 
slit of the collimator, and which consists of a tube about five. inches in length 
carrying a.t its further extremity a condensing lens of 1 j-inch aperture, is very con¬ 
venient, not so much on account of any increase iii the intensity of illumination, 
wliich is usually but slightly augmented by it, as because the source of light may 
then be employed with equal advantage at a further distance from the slit. 
Before commencing the observations a folding screen of three folds, lined inside 
with black velvet, and made of such a size as to rest in the outer groove of the lower 
base-board, is placed behind and on either side of the spectrometer, A circular 
aperture is cut in the middle fold, somewhat to the right of the centre, of sufficient 
size to permit of the passage of the illumination tube of the spectrometer, A second 
aperture of somewhat larger size is also cut at the same height in the middle fold 
near the left corner, and is provided with an easily moving door so that it may be 
closed when not required. Behind this larger aperture is placed the goniometer 
lamp described in the preceding communication, and which is shown in the back¬ 
ground in the illustration, from which the screen is omitted for obvious reasons. 
In order to adjust the prism and to measure its angle, the spectrometer is arranged 
with the illumination tube of the collimator directed towards or passing just through 
the larger aperture, so as to receive the light from the goniometer lamp. The door 
may be partially closed so as to shut ofi‘ most of the light from the observer wdiile 
adjusting the images of the slit to the cross wires. 
The prism is first adjusted and centred by means of the circular and rectangular 
motions provided for the purpose, and the angle is then measured in the usual 
goniometrical manner. For the adjustment a white background is an advantage, in 
