8 



OPTICAL INSTRUMENTS. 



composed of several pieces, as shown 

 by the front view E, where the 'lens is 

 built of ten pieces. These instruments 

 have been denominated by Dr. Brewster, 

 who suggested this division, Polyzonal 

 lenses. 



The following advantages of these 

 lenses have t been laid down by. Dr. 

 Brewster. 



1 . The difficulty of procuring a mass 

 of flint-glass proper for a solid lens of 

 great dimensions, is in this construction 

 completely removed. 



2. If impurities exist in the glass of 

 any of the spherical segments, or if an 

 accident happen to any of them, it^can 

 be easily replaced at a very trifling ex- 

 pense. Hence, the spherical segments 

 may be made of glass much more pure 

 and free from flaws and veins than the 

 corresponding portions of a solid lens. 



3. From the spherical aberration of 

 a convex lens, the focus of the outer 

 portion is nearer the lens than the focus 

 of the central parts, and, therefore, the 

 solar light is not concentrated in the 

 same point of the axis. This evil may, 

 in a great measure, be removed in the 

 present construction, by placing the 

 different zones in such a manner that 

 their foci may coincide. 



4. A lens of this construction may be 

 formed by degrees, according to the con- 

 venience and means of the artist. One 

 zone, or even one segment may be 

 added after another, and at every step 

 the instrument may be used as if it were 

 complete, without the rest of the zone 

 to which it belongs ; and it will contri- 

 bute, in the proportion of its area, to 

 increase the general effect. 



5. If it should be thought advisable 

 to grind the segments separately, or 

 two by two, a much smaller tool will be 

 necessary than if they formed one con- 

 tinuous lens. But, if it should be reck- 

 oned more accurate to grind each zone 

 by itself, then the various segments may 

 be easily held together by a firm cement. 



6. Each zone may have a different 

 focal length, and may, therefore, be 

 placed at different distances from the 

 focal point, if it is thought proper. 



CHAPTER III. Spectacles Periscopic 

 Spectacles. 



(13.) Spectacles* When two lenses 

 are mounted in a frame to fix before the 



* These instruments are said to have been in- 

 vented about the year J290. 



eyes, they are denominated spectacles : 

 the lenses are employed to render the 

 objects before the wearer more distinct. 

 The eye, which consists of a convex 

 lens,^ called the crystalline lens, re- 

 fracts the light proceeding from the 

 object placed before it in the same 

 manner as a convex glass : the image of 

 the object is formed at the focus of 

 the lens, where it is received on a 

 screen at the back of the eye; this 

 screen, called the retina, is an ex- 

 pansion of the optic nerve, which con- 

 veys the sensation of vision to the mind. 

 As the crystalline lens of the eye will 

 only produce distinct vision when the 

 focus is thrown on the retina, it is ob- 

 vious that should any defect occur with 

 respect to that organ, indistinct and im- 

 perfect vision will arise. Thus, if the lens 

 of the eye is not of a proper convexity 

 to bring the image on the screen, an in- 

 distinctness must ensue. This is the case 

 when the lens through age has become 

 flattened ; the image will then be thrown 

 beyond the retina, and thus convey an 

 imperfect representation of the object 

 to the mind. To obviate this defect, we 

 must make the rays pass through a glass 

 of sufficient convexity to assist the eye, 

 and enable it to form the image at the 

 required place, which is in this in- 

 stance done by shortening the focal 

 distance of the crystalline lens of the 

 eye. If, on the contrary, the eye should 

 be too convex, or short-sighted, as 

 is often the case with young persons, 

 then the image will not be formed at a 

 sufficient distance from the lens of the 

 eye to reach the retina, and thus imper- 

 fect vision of distant objects is produced. 

 To remedy this defect concave lenses 

 must be resorted to, in order to diverge 

 the rays before they enter the eye, and 

 thus lengthen the focus of the crystal- 

 line lens to form an image on the retina. 

 When the eyes are not directed near 

 the centre of the spectacle-glasses, the 

 obliquity of their surface to the rays will 

 be increased, so as to occasion a con- 

 fused appearance of the object. A great 

 portion of this confusion is removed 

 in the spectacles now usually made, 

 when compared with those formerly em- 

 ployed, whose size, being very large, 

 augmented the imperfection ; for it 

 may be observed that when objects are 

 seen through spectacle-glasses, no 

 more of the glass is employed at one 

 view than a portion equal to the size of 

 the pupil of the eye ; this on an average 

 may be reckoned at the eighth of an 





