7t>o 



THE POPULAR SCIENCE MONTHLY. 



first nearest approach the radius of either 

 star's nebulous envelop was greater than 

 the distance between the centres of the two 

 bodies, the atmospheric resistance would 

 tend to transform the parabola or hyper- 

 bola, in which the body was moving, into 

 an ellipse. Each subsequent return would 

 shorten the period, until, in the process of 

 cooling, the stellar atmosphere had so far 

 contracted as no longer to involve any part 

 of its companion's. 



It would be an interesting question 

 whether some of the double stars, whose 

 apparent distance apart has seemed too 

 great to justify the hypothesis of a physical 

 connection, may not afford other instances 

 of motion, either as parabolas or hyperbo- 

 las. 



Shool-Life and Eyesight. In a commu- 

 nication to the Mechanics' Magazine, Prof. 

 Liebreich describes the injuries to the eye 

 incident to school-life, pointing out their 

 causes, and the means to be adopted to 

 avoid them. The changes in the functions 

 of the visual organ, developed under the in- 

 fluence of school-life, are three in number : 

 First, decrease of the range of vision ; sec- 

 ond, decrease of the acuteness of vision ; 

 third, decrease of the endurance of vision. 



Decrease of the range, short-sightedness 

 (myopia), is developed almost exclusively 

 during school-life rarely afterward, and 

 very rarely before that time. There is of- 

 ten an inherited predisposition to become 

 short-sighted, and this is developed during 

 school-life, more or less, according to cer- 

 tain external conditions. It is a common 

 notion that short-sighted eyes are the most 

 durable. This is founded on the fact that 

 such eyes can see near objects distinctly 

 without the aid of glasses, at an age when 

 normal eyes require the assistance of con- 

 vex lenses. But this is no proof of their 

 durability. On the contrary, a high degree 

 of short-sightedness is a diseased condition, 

 caused by anatomical changes in the mem- 

 branes of the eye, which involve a greater 

 tendency to serious complication than the 

 normal eye. Short-sightedness exerts an 

 injurious influence on the general health by 

 inducing the habit of stooping ; and, from 

 a national point of view, its increase is to be 

 considered a serious evil. 



Decrease of acuteness of vision (ambly- 

 opia) is a serious condition, generally the 

 result of positive disease of the eye, which 

 may exceptionally be induced at school. 

 Amblyopia of one eye only, is, however, 

 often produced by unsuitable arrangements 

 for work, which disturb the common action 

 of the two eyes, and weaken the eye which 

 is excluded from use. 



Decrease of endurance (asthenopia) is a 

 frequent affection, that has destroyed many 

 a career, prevented the development of 

 many a fine intellect, and deprived many 

 of the fruits of their laborious exertions. 

 It arises principally from two causes : the 

 first is a congenital condition, called hyper- 

 metropia, which can be corrected by con- 

 vex glasses, and which cannot, therefore, be 

 laid at the door of school-life ; the second is 

 a disturbance in the harmonious action of 

 the muscles of the eye a defect which is 

 difficult to cure, and which is generally 

 caused by unsuitable arrangements for 

 work. 



These three anomalies all arise from the 

 same circumstances, viz., insufficient or ill- 

 arranged light, or from a wrong position 

 during work. Where the fight is insufficient, 

 or badly arranged, we are obliged to lessen 

 the distance between the eye and the book 

 while reading or writing ; and we must do 

 the same if the desks and seats are not of 

 the right shape and size, and suitably lo- 

 cated. When the eye looks at a very near 

 object, the accommodating apparatus, and 

 the muscles which turn the eye, are brought 

 into a condition of extra tension, and this is 

 to be considered as the principal cause of 

 short-sightedness and its increase. If the 

 muscles of the eye are not strong enough to 

 resist such tension for any length of time, 

 one of the eyes is left to itself; and, while 

 one eye is being directed on the ob- 

 ject, the other deviates outwardly, receives 

 false images, and its vision becomes indis- 

 tinct (amblyopia). Or, perhaps, the muscles 

 resist these difficulties for a time, become 

 weary, and thus is induced the diminution 

 of endurance (asthenopia). 



In order to pi'event these evils, the light 

 must be sufficiently strong, and fall on the 

 table from the left-hand side, and, as far as 

 possible, from above. The children ought 

 to sit straight, and not have the book 



