1889.] on the Solar Surface during the Last Ten Ye.irs. 499 



diameter is first traced upon a piece of drawing paper, which is pinned 

 to a board just rigid enough to hold the paper firmly, and then the 

 whole is clamped on to the eye-end of the telescope. The eye-piece 

 and board are each capable of fine adjustment, so that the sharpest 

 image of the sun may be made just to fill the lOJ-inch circle, and a 

 marked diameter of the picture is brought into precise coincidence 

 with the direction of the daily motion. The clock-work of the equa- 

 torial then keeps the image fixed in position on the paper, whilst an 

 accurate outline is traced of the umbra and penumbra of every spot 

 visible on the disk. When the sky is clear this outline can be made 

 as correct as the finest point of a hard pencil can delineate it ; and 

 even when, as so often happens, the transparency of the atmosphere is 

 changing every moment, a short time at the instrument will generally 

 enable the observer to verify the perfect accuracy of his sketch. The 

 details are then filled in as quickly as the nature of the sky permits, 

 each portion of the drawing being over and over again brought into 

 coincidence with the projected image, in order to detect and remove 

 the slightest difference between them. By this means the final picture 

 gives the advantage of all the best moments of seeing that occur 

 during the progress of the observation, and not merely the result at 

 one single moment, which maybe far from the best for definition even 

 on the finest day. Immediately the spots have been completed the 

 faculse are traced, and their details rejiroduced as nearly as possible. 

 By the advice of Prof. Stokes, P.E.S., ar red pencil is used in drawing 

 the faculse, and thus the difference between bright and dark markings 

 and their connection with each other, stand out much more boldly 

 than if the same black pencil were used throughout. Finally, when 

 the sky is good for definition, the general surface of the sun is care- 

 fully scrutinised for some time, and any peculiarities noted. 



The sun's image having thus been sketched and examined, the 

 drawing-board is re[)laced by an automatic sj)ectroscope of six prisms 

 of 60°, through each of which the light may be made to pass twice, 

 and to which may be added a Hilger-Christie half-prism, raising the 

 total available dispersion to about 36 prisms of 60°. The chromo- 

 sphere is measured with the slit radial, a dispersion of six prisms 

 being generally used ; and when the definition is very good, a sweep 

 is made round the limb, with 12 prisms and a tangential slit, to 

 study the forms of the prominences and the direction of the currents in 

 the chromosphere. Spot-spectra have been occasionally observed with 

 the same instrument ; but lately (in a room adjoining the equatorial 

 dome) a large grating has been mounted, with which it is proposed 

 to take daily photographs and eye-measurements of the spectra of sun- 

 spots. A heliostat and a 5J-inch object-glass of Alvan Clark are 

 used in connection with the grating spectroscope. The combination 

 of this solar observatory with an establishment supplied with a com- 

 plete set of self-recording meteorological and magnetic instruments, 

 affords a ready opportunity of comparing solar results with the daily 

 photographic records of terrestrial phenomena. 



