OPTICAL INSTRUMENTS. 



inch in diameter. Thus, 'we see how 

 small a portion is used for the purposes 

 of vision ; but as it would be tedious to 

 require the eye always to look through 

 a small aperture, the glasses are left of 

 a sufficient size to admit of a moderate 

 degree of motion ; and, as we require a 

 greater latitude horizontally than verti- 

 cally, their figure is made of an oval 

 form. 



In the selection of spectacle-glasses 

 great care should be used in examining 

 them, and the first point of importance 

 is the goodness of the material of which 

 they are formed ; this should be free 

 from all veins or small bubbles, for if 

 one of these occur in the portion 

 through which we look, it will greatly 

 impair the eyes. The next circumstance 

 is the colour of the glasses ; the best 

 adapted for general purposes is a pale 

 blue. The figure of their surfaces should 

 be perfectly spherical, for if they are 

 curved more in one direction than in 

 another, they will injure the sight, unless 

 they are cylindrically formed, as for some 

 particular disease. The polish should be 

 clean, and free from flare, which too 

 often arises from the manner in which 

 they are usually polished on heteroge- 

 neous surfaces, producing what is 

 technically termed a curdled glass. 

 See the method of grinding and polish- 

 ing lenses, described in (48). 



(14.) Dr.Wollaston, in order to allow 

 the eyes a considerable latitude without 

 fatigue, invented a peculiar form of 

 glasses, called by him periscopic, from 

 two Greek words signifying seeing about; 

 their form is that of a meniscus with 

 the concave side always turned towards 

 the eye. When they are intended 

 for long-sighted persons, or old age, 

 the anterior surface, or that next the 

 object, is formed spherically convex, 

 with a curve deeper than the concave, 

 so as both to gain the required power, 

 and compensate for the divergency oc- 

 casioned by the concave side ; this form 

 is shown at A, (fig. 14.). The peri- 

 Fig. 14. 



scopic form employed for correcting 

 the defect of a short or near sight is 

 shown in section at B, having its an- 

 terior surface convex, as in the former 

 case ; but here the concavity on its pos- 

 terior side is increased to procure the 

 required divergency, and compensate for 

 the convex side. 



CHAPTER IV. Telescopes Common 

 Astronomical Telescope Day Tele- 

 scope Dynameters. 



(15.) A Telescope is an optical instru- 

 ment employed for viewing distant ob- 

 jects, by increasing the apparent angle 

 under which they are seen without its 

 assistance ; and hence the effect on the 

 mind of an increase in size, or, as com- 

 monly termed, magnified representation. 

 The construction of the Telescope is, 

 perhaps, one of the most important ac- 

 quisitions that the sciences ever attained, 

 as it unfolds to our view the wonders of 

 the heavens, and enables us to obtain 

 data for astronomical and nautical pur- 

 poses. 



The invention of this instrument is 

 somewhat uncertain, and is ascribed to 

 different individuals, as John Baptista 

 Porta, Jansen of Middleburg, and 

 Galileo. The time of its first construc- 

 tion was about the year 15 90. 



The simplest construction of this in- 

 strument consists of two convex lenses, 

 so combined as to increase the apparent 

 angle under which distant objects are 

 seen. If we take a convex lens, and 

 place it in a similar position to the object, 

 as that inyTg-. 7, and another of shorter 

 focus in the position Jig. 8, with a dis- 

 tance between them equal to the sum of 

 their foci, a telescope will be formed, 

 and the magnifying power will be in 

 proportion to the focus of the two lenses. 

 Let O (/o-. 15) be the object lens, and 

 suppose it 8 inches focus, and e the 

 eye lens, of 2 inches focus, the distance 

 between these two lenses must be ten 

 inches, if the object be at an infinite 

 distance, as a star ; but when the object 

 is terrestrial, the distance between the 

 two lenses must be increased to adjust 

 for distinct vision : on this account the 

 eye lens is mounted in a tube, sliding 

 within another tube in which the object- 

 glass is fixed, and, therefore, can be 

 drawn out for near objects. As the 

 size of objects is dependent on the angle 

 under which they are seen, the image 

 F, formed by the object-glass, in the 

 focus of the eye-glass e, will subtend 



