686 KEroHT— 1893. 



metrical figures on glass. They were intended to be placed in front of a telescope 

 foeussed to a distant point of light, according to the plan of Fraunhofer and 

 Schwerd. The result is a system of radiating lines, which consist of spectral 

 images of the point of light ; but if the same are viewed as above with the eye- 

 piece alone, the extending spectra mostly disappear, and more elaborate and finely 

 defined central figures are formed. 



10. Oil Wilsmi's Theory respecting the asserted foreshortening of the inner 

 side of the Penumhrca of the Solar Spots ivhen near the Sun's Limh, and 

 of the prohable thickness of the Photospherio and the Pemimhral Strata 

 of the Solar Envelopes. By E,ev. Frederick Howlett. 



For a considerable portion of the period of upwards of thirty years, during 

 which the author of this paper has maintained a more or less continuous record of 

 the solar spots — during, be it noted, three full successive periods of the maximum, 

 minimum, and intei-mediate conditions of solar-spot activity, and including some 

 thousands of careful and roughly micrometric observations — one point has not a 

 little excited his surprise, viz., to have scarcely in anyone undoubted instance been 

 able to verify the observation first made by Dr. Wilson, Professor of Practical 

 Astronomy in the University of Glasgow, as long ago as the months of November 

 and December 1769, as well as on, he aflirms, many subsequent occasions. 



The phenomena in question which Wilson claims to have frequently seen is, in 

 brief, this, that if a spot, when well on the disk, has its penumbra equally dis- 

 tributed on all sides of the umbra, the effects of foreshortening on the sphere, in 

 consequence of the funnel-shaped nature of a spot, will be that whenever a spot is 

 near the limb the side of the penumbra nearest to the sun's centre will be extremely 

 foreshortened, and that when very near the limb, not only the whole of the inner 

 side of the penumbra, but the whole of the umbra itself, will become invisible, the 

 outermost side of the penumbra alone remaining in sight. 



Wilson tells us (as recorded in the ' Philosophical Transactions' for 1774) that 

 he effected his observations by direct vision, using a small, and he says an excellent, 

 Gregorian reflecting telescope of 26 inches focal length, with a magnifying power of 

 112 linear. 



The author's observations were made by projecting the sun's image on a large 

 -screen nearly 5 feet by 4 feet, using a small but excellent refractor by the elder 

 Dollond of 3" inches aperture with 46 inches focal length, with magnifying powers 

 of from 80 to 200 times linear. 



When using power 80, with the screen placed 4 feet 3 inches from the eyepiece, 

 the projected image of the sun has a diameter of 32 inches, so that, consequently, 

 each inch of the image is equivalent to just about 60" of celestial arc, and which 

 is the scale on which the author's drawings are usually made. A sort of micro- 

 meter, moreover, consisting of a small disk of glass ruled off into the two-hundredths 

 of an inch, is placed in the focus of the eyepiece, so that the divisions on the glass 

 disk are distinctly seen projected also on the screen, each exactly half an inch 

 apart. 



When a power of 200 linear is used the sun's image is seen 6 feet 4 inches in 

 diameter on the screen when placed at the same distance from the eyepiece as before 

 mentioned, and when each minute of arc thereon measures 2h inches, so that, in 

 fact, in such enlarged images seconds of arc can be readily measured by a pair of 

 common dividers. 



Now as regards Wilson's observations — with which the author professes him- 

 self to be most strongly at issue, especially when spots of any considerable magni- 

 tude are concerned — nothing could be stated in a more exact, cautious, and circum- 

 stantial manner. And it is, in all probability, in consequence of this that the 

 phenomena Wilson claims to have seen have been handed do-wTi as facts (without 

 their having been adequately verified or disproved) in almost all works on physical 

 astronomy to the present day. 



The author, however, had the honour of calling the special attention of astro- 



