Lessou XL 







the surfaces of the retractive body, we 



ree and direct! >n of the 



retraction : We "':>' bring 



| s of light to u forus by means of a 



ive body, the surface of which pre- 



parts with ditfereiit degrees of 



obtkra 



/'What are the usual forms 

 :< lenses? 

 p. There are seven kinds : 1st, the 



b c d c f s 



1- g. 38. 



il kind, made of a globe of glass 

 (a) tilled with water, or solid ; 2nd. the 



T, or one that has both external 

 surfaces convex (6) ; 3rd, the }>l>ino-c<mvfx 

 (r). which has one plain and one convex 

 iurface ; 4:h. the In-t-onctn'c (</), or one 

 that has both stir aces concave externally ; 

 5th, the plano-cancnve (e), or one that has 

 one surface plain and the other concave ; 

 6'b, theowfvm' court /. having one surf.ice 



and the other convex, this kind 

 is called a irnici (/), Iwcause it is like 

 a little moon ; 7th, another trm of cnn- 

 cavo-comvex (y). which is also considered 

 to be a mc*i*cu*. The 2nd (6). 3rd (c), 

 and 6th (/). are thicker at the centre 

 than the edges, am! 



bows. rgent lenses are thinner 



in t)u middle than at the edges, such a 

 :,th (< ). and 7:li (y). 



;nds of 

 glasses used in optic*? 



/'. Ye*. - v. ml ; nut with the 

 tion of prisms or r glasses, and 



plain tablets :s are not s 



rally used, such as fr poLris 



do you mean by the 

 polarisation of light? 



1'. The * ;>ui ray of light 



:n th. 



otlur in ;> properties; the most < 

 of these U that of pr< .nr in 



several ways from a ray which appears to 

 be colourless. 



that a ray of ligut has any *.- 



/'. 



t of the proper : 



that of imparting c<>lonr t * bodies 

 will, pv ir rather absurd 



p. rsons to he.ir that a green leaf 



to a cerutiu 



extent it is true. The eo.ours of all the 

 objects that surrouud us are due to :he 

 I.:;/, the colour of a substance 

 resultin >ur!ace being adapt i-d to 



reflect its peculiar colour. 



136. T. How is the infliien 

 the produeti. n of colour ino. 



/'. Bj refraction; and this is r 

 dcmonstra'-ed by me.ms of a prism of 

 glass, or by the following simple i 

 uient. [Kxjwriment i?'J J Place a pirc- 

 of paper upon the table, and 

 then place a wineglass, with 

 a stem cut (as in the accom- 

 panying figure) into ni: 

 more planes, upon the ei : 

 the paper, taking c;.i 

 arrange them on the table so 

 that a ray of light from the 

 sun shall pass through th- 

 stem, just as I do now. [Does 

 so, and the paper is found t 

 be coloured.] You see that 

 Fig. 39. there are several e 

 upon the paper. First you s e there it. 

 vuili't, which is a compound colour; then 

 nnother compound ; then blue, an 

 colour: then green, a com: 

 an original ; orange, a compound; 

 and ]' ii. > that these 



Off i::i<-.ju;il 



and so ' to say 



Is. If 



.1 ray of lii, r 't is per- 

 formed in a 



ii.rror, an ol 



i is called the *pcctr*m, 

 and which exhibits the prisn 

 we have seen. 



/'. \vi,at do you mean by pris- 



rs? 



/'. Tn. y are the simple colour* of tbe 

 -.cause they 



1 by decomposing a ray of 

 light !>y means ot' a nri*m. 



/'. Has thit knowledgr been 

 to any navful pur- 

 POM- :- 



A Yes, to several; and the im 



nse is chiefly 



