76 



t NDULATO&Y rOROBB. LIGHT. 



[MAGNIFYING GLASSES, KTC. 



so that our readers may more fully understand tlio 

 principles on which > 



In our remarks mi the structure of tin- eye,* wo men 

 tiom-l that the cornea is of a convex f..rm, like an 

 ordinary convex glass lens. It ]M>rfi>rms that office to 

 the eye whirh a lens does to a microscope or telescope; 

 that is, it collects the rays of light juusing from any 



t. Each part of the eye, however, is exactly 

 adapted to its speciid purpose, and any disarrangement, 

 no matter how tritlini,'. is followed by inconvenience. 



i in enrlv yos of some persons have the 



T too flat : Hii<! 



defects at' ' / cm ploying glasses orspectnc 



an opposite character. Thin a highly convex cornea 

 requires a concave Ions ; and one too Hat is remedied hy 

 the employment of a convex glass. When the defect 

 rises from the cornea not having sutlioient convexity. 

 Ih.-'i th" rays of light received and refracted by it, have 

 their focus behind the retina. The effect is very similar 



i.it found when the lenses of the magic lantern, 

 A T c., are out of focus. The result, of course, is, tli 

 person so affected suffers from great indistinctness of 

 vision. This will be better understood by reference to 

 the annexed diagram, in which it is supposed that the 

 cornea has not sufficient convexity for the ordinary pur- 

 poses of vision. 



Fig. 64. 



In Pig. 64, a represents the cornea, to which rays of 

 light are supposed to l>e passing, unimpeded, from dx. 

 Owing to the want of proper convexity, they pass on 

 from t f, and do not reach the focus till they arrive at 

 r, a point beyond the retina, b. If, however, a lens, g, be 

 interposed between a and dx, then, hy its convexity, 

 it brings to a focus all the rays on the retina of the eye 

 at b. The difficulty experienced by all persons so affected, 

 is to find what is called their "sight;" that is, the focal 

 distance of the lens, g. This can only be done by trial ; 

 nnd when successful, the result will be due to a choice of 

 a glass whose focal point exactly falls on the retina when 

 the spectacles are placed in their proper position before 

 the eyes. The use of any pair of spectacles is, however, 

 limited to special purposes; because, unlike the eye, they 

 cannot accommodate themselves to the change of position 

 or distance, in which objects are ordinarily found. A 

 vc ry ingenious arrangement, however, is sometimes cm- 

 d. Instead of using one glass in each eye-hole of 

 the spectacles, two, of different focal distances, are placed, 

 so that four, instead of two lenses are employed. The 

 two at the bottom of the spectacla's are used for reading, 

 sewing, and other purposes requiring close vision. The 

 npper lenses arc useful for viewing distant objects ; and 

 such a combination is of great value to elderly persons. 

 We i-innot help remarking on the imperfection of all 

 human contrivances when put into comparison with those 

 of the Creator. The eye, in its perfect state, can in- 

 stantaneously adapt itself to any purpose for which we 

 require it ; all the intermediate steps of its arrangement 

 nr > unperceived by us. lint despite the skill of our best 

 workmen, all attempts at imitating this are eminon'lv 

 faulty, partly from our ignorance of the necessary 

 mechanical arrangement, and still more so from the 

 absence of vitality in the instruments we employ. 



We may now examine into the opposite defect of sight 

 namely, that in which the eye is too convex ; and we 

 need scarcely say, that the remedy is exactly the con- 

 trary to that we have just proixwed. This will IMS at once 

 understood by inspecting the following engraving. 



In Fig. (>~>, we presume that the rays are proceeding 



i fpiin an ol.ji , i. /.-. t., the c. irnea, a. The cornea lining too 



convex, converges the rays too rapidly, and they therefore 



at c, between the cornea, a, and retina, b. 



8f **tl, p. 48. 



If, however, a double concave lens, g, be interposed be- 



i ii and i/j-, then the rays impinging on i / will not 

 -e except at Ii (which is the retina;, providing tho 

 lens is chosen of tho proper focal length. 



. 65. 



In tho absence of glasses, individuals affected in the 

 manner to which wo have been adverting, endeavour to 

 accommodate either objects or the eyes, so as to par- 

 tially overcome tho incmneiiicnce to which tin . 

 subject. Thus a short-sighted, person, or one whose eye 

 is too convex, sees better if the objects are placed close 

 to the eye ; whilst those whose cornea is too tl. 

 the object to some distance from the eye. In either 

 case, a temporary alteration of the visual focus is thus 

 effected. 



MAGNIFYING AND BURNING GLASSES. 



\\'i: have classed under this head two instruments, whose 

 properties arise from a similar cause that of simulta- 

 neously concentrating the rays of light and hc:it when 

 two forces are present at the same moment. Of 

 course our remarks are now chiefly directed to those 

 circumstances in which solar or analogous influences are 

 experimented on. In no case do we lind the light of the 

 sun unassociated with calorific effects; and as each force 

 is subject to the same laws, these effects may be con- 

 sidered together. 



In a former page,t we stated, that light, emanating 

 from a distant source, does so in what may practically be 

 considered as parallel rays. As such, the results are 

 nearly coincident in a point called the focus. The 

 effect varies according to the distance between the centre 

 of tho lens and the focus of the lens. If, however, the 

 lens have a large curvature, and consequent distant focus, 

 it will have the power of collecting from a greater area a 

 larger amount of each radial force. We therefore find 

 that the magnifying and burning power of any lens are 

 not coincident. Added to this, we must take into ac- 

 count the effects of spherical aberration. The inlluences 

 which may control the latter source of disturbance have 

 been already discussed in our previous pages,J to which 

 we must refer our readers. 



Some extraordinary statements have been made in 

 reference to the power of different kinds of reflectors and 

 refractors, which have been employed for the purpose of 

 concentrating the solar rays of light and heat. By ! 

 mirrors rejecting these forces, platina, quartz, and 

 other refracting substances have been easily melted. 

 The most successful experiments have been tried by 

 means of retlectors which have been built up in segm> 

 forming part of a sphere or ellipse. Such are i 

 cll'cctive when arranged as reflectors of a parabolic form. 

 ( lur own experience leads us to believe that there is a 

 limit which entirely prevents the employment of solid 

 jlass lenses for such purposes. Experiments we have 

 iried with a double convex glass, twenty-eight inches in 

 liameter, and about six inches thick, showed a loss of 

 light and heat, by absorption, tvc , which rendered the 

 lenses practically useless. In such instances, reflectors 

 of the same size produced a far greater effect. 



It has often been stated, that Archimedes, by means 

 if burning-glasses, destroyed the fleet of his enemies. 

 Whilst wo congratulate him on his supposititious \ictory, 

 we may prudently doubt its cause. Suffice it to say, 

 hat modern science, with all i; : and adjn 



' - not, as yet, means at all equivalent to those which 

 would be essential to the success of the hero we h.nc 

 named. 



If convex glass lenses of largo size are employed, it is 

 advisable that they should be formed of blown glass of 

 t Bee ante, p. 44. } Ante, p. 44. 



