30 LOCKYER. — USE OF THE SPECTROSCOPE. 



Instructions for use of Instrument B, 



Three sets of direct-vision prisms are supplied with this spectro- 

 scope. When all three sets are used the dispersion wiU be suffi- 

 cient to enable it to be used for the Sun in the same manner as 

 instrument A (except for the least refrangible portion), when some 

 of the foregoing instructions will apply. 



It has been, however, provided chiefly for observations of the 

 Aurora, and when used for this purpose only one set of prisms (as 

 packed in the box two sets are mounted) and the lower power eye- 

 piece should be employed. 



One method of measurement which can be employed by this 

 instrument is as follows : — The adjusting screw with scale nearest 

 the eye, for reference called the " reading screw," is set to the cen- 

 tral division marked 10 on the paper scale. The other similar 

 adjusting screw furthest from the eye, for reference called the 

 " bending screw," should be adjusted so that the needle point visible 

 in the field of view is coincident with some definite line in the 

 green, say the green line in the aurora. 



Measures of auroral lines right and left of this line can then be 

 made and recorded, and these measures can be compared with 

 those of the lines of other spectra, either simultaneously by means 

 of the reference prism, or afterwards. 



A variation of this method will be to set the reading screw to 

 10, and the bending screw so that the solar line h is adjusted to 

 the needle point. Solar positions can then be read off before the 

 auroral observations are made, and a map prepared on which the 

 positions of auroral lines can be at once recorded. 



Spectkoscopic Work. 



Scales prepared on Mr. Capron's plan, together with forms for 

 recording positions, also accompany the instrument. In using 

 these carefully insert the principal solar lines in their places on 

 the forms, as taken with a fine slit, and keep copies of this scale 

 for use. If the slit opens only on one side, note on scale in which 

 direction the lines widen out, whether towards red or violet. 

 AJso fill up some of these forms with gas and other spectra, as 

 taken at leisure with the same instrument and scale. 



When observing close the slit (after first wide opening it), as 

 much as light will permit, and then with pen or pencil record the 

 lines as seen upon the micrometer scale on the corresponding part 

 of the form, and note at once relative intensities with Greek 

 letters, a, ^, &c. (or numbers). 



Reduce at leisure line-places on scale to wave lengths, and note 

 as to each line the 'probable limits of instrumental error. 



In case the auroral spectrum is so faint that the needle point or 

 micrometer scale is invisible, half of the field of view may be 

 covered with tinfoil, with a perfectly straight smooth edge running 

 along the diameter of the field, in perfect focus, and parallel to 

 the lines of the spectra. The reading screw being set to 10, the 



