134 Intelligence and Miscellaneous Articles, 



rocks, which are foreign to tho island — chalk, chalk-flints, oolitic 

 limestone, oolitic breccia, dark limestone of Calciferous-sandstone 

 age, quartzites, gneiss. &c., some of which closely resemble the 

 representatives of these formations on the east of Scotland, and 

 have doubtless been derived thence. From this they infer that, 

 while Shetland was glaciated by the Scandinavian mer de glace, 

 Orkney was glaciated by the Scotch ice-sheet, the respective ice- 

 sheets having coalesced on the floor of the North Sea and moved in 

 a north-westerly direction towards the Atlantic. 



They also found abundant fragments of marine shells in most of 

 the Boulder-clay sections, which are smoothed and striated precisely 

 like the stones in that deposit. They conclude that these organisms 

 lived in the North Sea prior to the great extension of the ice, and 

 that their remains were commingled with the moraine profmde 

 as the ice-sheet crept over the ocean-bed. From the marked 

 absence of shell-fragments in the Shetland Boulder-clay, they are 

 inclined to believe that much of the present sea-floor round that 

 group of islands formed dry land during the climax of glacial cold. 



XXII. Intelligence and Miscellaneous Articles. 



AS APPLICATION OP ACCIDENTAL IMAGES. BY J. PLATEAU, 

 MEMBEB OP THE EOTAL ACADEMY OF BELGIUM. 



WHEN, on a fine night, with the naked eye we gaze at the full 

 moon near the summit of its course, it is impossible for us to 

 picture to ourseVes that a distance of more than 80,000 leagues 

 separates us from it : in spite of ourselves we judge it to be at a 

 relativelv very short distance. But what is that distance ? It seems, 

 at first sight, very difficult to estimate : the thing is nevertheless 

 possible: and this is how it may be done : — 



The absolute size which we attribute to an accidental image is, 

 vou know, proportional to the distance of our eyes from the surface 

 upon which we project that image. This results from the fact that 

 the image is due to a modification of a determined portion of the 

 retina, so that it subtends a constant visual angle. Besides, the 

 proportionality in question has been verified by Father Scherfer by 

 means of direct experiments*. If. for example, after contemplating 

 a small red disk placed on a sheet of white paper we cast our eyes 

 upon another part of the paper in order to observe there the green 

 accidental image, this will exhibit the same size as the small disk ; 

 but if we move the white paper gradually nearer to our eyes, we 

 shall see the green image diminish proportionally in diameter : if, 

 on the contrary, we turn our eyes to a rather distant wall, the image 

 will appear considerably enlarged. More precisely, the absolute 

 maamitude which we attribute to it is proportional to the distance 

 at which we picture to ourselves the surface on which it is pro- 

 jected. 



* Imtitv.tionum opticarum partes quatuor (Vienna, 1775), pais 1, 

 caput II. art. iii. § 99. 



