Conversion of Cameos into Intaglios, Sfc. 107 



which has obtained for it the name of hammered agate. 

 The surface on which these cavities appears, is a section of 

 small spherical aggregations of siliceous matter like abc in 

 Fig. 22, which present exactly the same phenomenon, aris- 

 ing from the same cause as the knots in mahogany and other 

 woods. 



The very same phenomenon is often seen in mother-of- 

 pearl. Indeed, it is so common in this substance, that it is 

 almost impossible to find a mother-of-pearl counter which 

 seems to have its surfaces flat, although they are perfectly 

 so when examined by the touch. Owing to the refraction 

 of the light by the different growths of the shell lying in 

 different planes, the flattest surface seems to be inequal and 

 undulating. 



One of the finest deceptions which we have ever met with, 

 arising from the disposition of light and shadow, presented 

 itself on viewing through a telescope the sui'face of a grow- 

 ing field of corn, illuminated by the sun when near the hori- 

 zon. This field, on Sir Walter Scott's estate at Abbotsford, 

 was about two miles distant, and was divided into furrows, 

 which were directed to the eye of the observer, as shown in 

 Fig. 23, where A B, C D, E F, represent the furrows. These 

 furrows are of course depressed, and the growing corn rises 

 gradually from two adjacent ones towards the middle m n, 

 op, so that the surfaces A)«C, CoE were convex. The 

 drills of corn on the highest summits mn, op, caught the 

 rays of the setting sun, which shone upon them very oblique- 

 ly in the direction S s, and illuminated their summits laterally, 

 while the furrows A B, CD, E F, were in shadow. The 

 consequence of this disposition of the light and shade was, 

 that the whole field seemed to be trenched, and the corn to 

 be growing in the trenches as well as upon the elevated beds 

 between them. The half furrow ABnm being shaded on 

 its edge A B, and illuminated on its edge m n, became the 

 elevated part of the trenched ground, while the other half 

 mn CD appeared the sunk part, in consequence of the side 

 m n being illuminated, and its other side C 1) in shade. At a 

 certain period of the day, this deception did not take place, and 

 it was dispelled the moment the sun had set. This very 



