March 4, 1875] 



NATURE 



351 



its glass the flame is drawn downwards by a powerful 

 current of air which circulates in a lower pipe to which 



M. NuiUer's work.) 



the tube B is fitted. Owing to the draught the burner 

 does not become heated, and the hand may be placed 

 upon it without being burnt ; the robe of a 

 dancer may without danger brush it, since 

 the flame is produced in an enclosed space. 

 If the glass A is accidentally broken, 

 the burner E, mounted on a pivot, is 

 lowered at C, and by this movement sets 

 in motion a small valve, which shuts off 

 the gas, and the light is thus put out. 

 The jets of the footlights are arranged in 

 series of twelve, and number in all 120. 

 These 120 lights may be raised to the 

 height of the stage, or lowered underneath 

 the prompter's hole, below the flooring of 

 the front stage, by a mechanism which 

 draws them all together, and which two 

 men can easily move. 



By a very ingenious arrangement all 

 the lights in the theatre can be lowered 

 suddenly so as to produce a night-eftect, 

 without the least danger of any of them 

 being extinguished. 



One of the most important applica- 

 tions of science in the New Opera is 



the use made of electricity, which we shall describe in 



another article. 



{To be continued.) 



ENGLISH GOVERNMENT ECLIPSE EXPE- 

 DITION, 187s 



Instructions to Observers.* 

 l.— Spectroscopic Observations— Objects to be attained. 

 The objects to be attained are mainly the deter- 

 mination, so far as may be possible, of the chemical 

 constitution of the chromosphere and of the coronal atmo- 



* Drawn up by the Eclipse Committee of the Royal Society. 



Fig. 4. — Gas-jet. 

 with reversed flame, 

 used in the footlights 

 of the New Opera 



sphere ; of the height to which the various vapours ex- 

 tend from the photosphere, and of the order in which they 

 thin out. It IS anticipated that the chroinosphcre at 

 all events, may be very rich in ultra-violet rays. The 

 solar spectrum has already been photographically com- 

 p.iicd with metallic vapours from G some distance out- 

 wards. The operations, therefore, will be mainly photo- 

 graphic, glass being employed as little as possible to 

 produce the necessary dispersion, and replaced by quartz. 

 The attack is twofold, spectroscopes being used in con- 

 junction with telescopes for obtaining line spectra, and 

 prismatic cameras being employed for the purpo'se of 

 obtaining images of the chroinosphere and coronal atmo- 

 sphere built up by the rays emitted by its various consti- 

 tuents. The prismatic camera will probably give the 

 best results with regard to the height and order of 

 the various layers, while the general nature of the spec- 

 trum beyond H, i.e. whether it is continuous, channel- 

 spaced, or lined, will be best determined by the ordinary 

 spectroscopes. 



Adjustment of Spectroscopes. 



Take out camera, and determine focal point for blue 

 rays by receiving image of sun on ground glass, and by 

 using solution ot sulphate of copper in ammonia in front 

 of object-glass. (The strength of solution to be deter- 

 mined beforehand such that no light less refrangible than 

 G should pass at all, and that the centre of gr.wity of 

 spectrum is H, or outside it.) 



To determine focus of collimator, reinsert camera and 

 move sliding portion of colliinator attached to slit-plate 

 till the lines of the spectrum at or outside H are clearly 

 delined. 



All prisms to be set for minimum deviation of H. 



To find proper distance of slit-plate from telescope, 

 lhi;ow image of sun on, so as to cover half the slit, and 

 adjust the spectroscope to such a distance that the boundary 

 of the spectrum of the photosphere at H is perfectly hard. 



Photographs should be employed for ascertaining the 

 focus ; the slits to be clean, and adjusted so that at 

 least three lines be seen between the two H's. No photo- 

 graph need be examined which will not bear a mag- 

 nifying power of ten times. It must be remembered that 

 a difference of i-ioooth of an inch is of importance in 

 such adjustments. The best definition with the dispersion 

 employed will be attained when the line in the middle 

 between the H lines is seen double. 



The hardness of the sun's limb to be determined photo- 

 graphically in the same manner. 



If power to incline the plate is obtained, the part of the 

 plate to receive the more refrangible rays will, of course, 

 be nearer to objective of camera, as in the case of all non- 

 achromatic lenses. The angle to be determined by ex- 

 periment. The spectrum should fall on the plate so that 

 G falls close to one edge, the central and other portions 

 of the plate being reserved for the more refrangible end of 

 the spectrum. 



Care must be taken that the axes of the colliinator and 

 of the telescope be coincident. 



Adjustment of Prismatic Camera. 



This instrument is to be adjusted like an ordinary 

 spectroscope by means of collimator placed in front of its 

 prism. By application of external collimator, the prism 

 is to be set to minimum deviation of H, the hydrogen 

 line near G falling near one edge of the plate. 



Before this instrument is put on the telescope, the prism 

 thus adjusted should be taken off and perfect parallelism 

 of the tubes obtained by observing the images of the sun 

 or star. 



The subsequent inclination of the two axes will be 

 determined by taking photographs of spectrum with or 

 without a collimator, so that ring corona near G will be 

 the least refrangible portion of spectrum on plate, while 

 the sun falls on the_ steel plate of the telescope to which 



