54- TEE POPULAR SCIENCE MONTHLY, 



marshy plain almost level with the sea in about four days, commenc- 

 ing on September 30, 1538. 



The whole hill is, therefore, the product of one eruption. It is an 

 interesting fact that no stream of lava was developed. It would seem 

 that the explosions were so intense that the fluid rock was entirely 

 broken up and ejected in a fragmentary condition, of which there are 

 great quantities forming the slopes. 



The cone of Vesuvius proper, fifteen hundred feet high above the 

 lowest edge of the crater of Somma, has entirely been built up of the 

 ejecta thrown out at the time of and since the memorable eruption of 

 A. D. 79, in which Ilerculaneum, Pompeii, Stabia, etc., were destroyed. 

 Besides the bulk of the mountain now seen, we must not forget the 

 vast quantity that has been required to fill up the crater of Somma, 

 much enlarged by the eruption spoken of. It is said that no lava ran 

 from Vesuvius till the tenth century ; this probably would be explained 

 by the fact that all the earlier streams were occupied in filling up the 

 great crater. Science- Gossijx 



-- 



EYES AND SCHOOL-BOOKS. 



By Peofessor HEEMANN COIIN. 



t 



IT was formerly considered, and some recent text-books have re- 

 j^eated the error, that the qualities of near-sighted and long-sighted 

 eyes were opposed. The investigations of Professor Donders, of 

 Utrecht, Lave, however, shown that not only is long-sightedness not 

 the opposite of near-sightedness, but that the two defects may be 

 associated in the same individual. The real opposite of short-sighted- 

 ness, according to Professor Donders, is over-sightedness. He dis- 

 tinguishes three kinds of eyes : 1. Those whose axis is of the proper 

 length from front to rear, normal-sighted or emmetropic {ev fi^rpo) (hxp, 

 seeing at the right distance) ; 2. Those whose axis is too long, short- 

 sighted, or myopic (from (iveLV, to Wmk, from the habit common to 

 near-sighted persons of partly closing the eyelids in looking at distant 

 objects) ; and, 3. Those whose axis is too short, over-sighted, or hyper- 

 metropic, seeing heyond the measure. To see at a distance, the em- 

 metrope needs no glass, the myope a concave glass, the hyperope a 

 convex glass. 



All three kinds of eyes may become far-sighted or old-sighted as 

 their near vision becomes weaker in old age. This kind of far-sight- 

 edness is no more a disease than the turning gray of the hair ; it 

 depends upon the diminished force of the muscle that curves the 

 crystalline lens for near vision. 



