438 Sim Viewing and Drawing. 



interposed disc of glass, so that it will be found expedient 

 frequently to wipe it. The Huygenian lenses themselves 

 always remain remarkably free from any such condensation, the 

 cause for the difference resulting, probably, from the different 

 qualities of the glass itself. 



It was observed above that the apparent size of any solar* 

 object visible on the screen was smaller or larger according to 

 the distance which intervened between the screen and the 

 eye-piece ; and it was eventually found that when a power of 

 120 linear was employed, and when the screen was placed just 

 five feet two inches from the eye-piece, one of the gradua- 

 tions cf 30" of arc, measured upon the screen exactly one inch. 

 Hence, of course, half an inch upon the screen was equivalent 

 to 15" of arc, and this scale is, perhaps, as convenient and 

 instructive as any, for the purpose of depicting solar pheno- 

 mena, which may be comfortably copied at once off the screen 

 upon transparent tracing paper, ruled across at regular 

 intervals with faint lines, forming squares half an inch in size.* 

 Lead pencils of the best quality should be employed in 

 delineating the solar spots, the observer sitting in his camera 

 obscura, with his back of course to the window shutter, and 

 holding his tracing-paper somewhere or other within the cone 

 of rays which diverge from the eye-piece, and which affords 

 abundant illumination for the purpose in band. 



When using the ordinary Huygenian astronomical eye-piece 

 in connexion with the screen, the projected solar image will 

 be seen reversed, but not inverted as when the sun is viewed 

 by direct vision. The disposition of the image is, however, 

 otherwise affected by projection (it is turned inside out, as it 

 were), and in order to correct all optical freaks and represent 

 the phenomena as they would really appear in the terrestrial 

 eye-piece, it will be necessary both to reverse and also invert 

 the drawings on the tracing-paper, gumming them down (if it 

 is wished to preserve them) upon uniformly-sized sheets of 

 very pale stone-coloured drawing-paper, which can be bound 

 up into volumes for reference. 



The drawings are thus preserved from any possibility of 

 being smudged, whilst at the same time the slightest mark 

 of the pencil will be visible through the transparent 

 tracing-paper. The faculas — a very delicate feature t6 deli- 

 neate — should be carefully executed in Chinese white with a 

 camel's hair brush, avoiding a too abrupt and harsh outline. 

 But inasmuch as the Chinese white is apt to be nearly oblite- 

 rated when treated with the gum-arabic, the faculse should 



* Messrs. Droosten and Allen, of the Strand, London, makes up convenient- 

 sized block-books of this tracing-paper, interleaved (as is necessary) with a white 

 opaque paper, as a contrast to the pencil marks. 



