156 



OPTICS. 



139. T. What do you mean by achro- 

 matic ? 



P. A substance without colour, the 

 term bt- ing derived from two Greek words. 

 a (a) friratiri, and Kroma (XpaJ/ua), 

 colour. The term is applied to these 

 lenses because the coloured fringes and 

 other defects observed in images formed 

 by a single lens are removed. 



GENERAL QUESTIONS ON LESSON XI. 

 1. What do you mean by refraction ? 



Lesson XII. 



2. Kxphiin how refraction takes place, 

 and illustrate ir by simple experiments. 



.'?. Does refraction alwais take place 

 when a ray of \\r\\t passes from a rarer 

 to a denser medium ? 



4. Do all substances possess the same 

 power of refraction ? 



5. Describe the various kinds of lenses. 



6. What is meant by polarisation of 

 light ? 



7. What are the colours of the solar 

 spectrum ? 



LESSON XII. 



THE organs of vision are usually described under the head of optics, but I think it 

 better that they should be described when treating of anatomy, and we will now enter 

 upon the consideration of some Optical Instruments. 



Optical instruments are so various, that it would be impossible to describe one-fourth 

 of them in our volume, we shall therefore only consider the most common, com- 

 mencing with the Telescope. 



QUESTIONS. 



140. 7'. What is the most simple kind of refracting telescope ? 



P. The astronomical, which consists of two convex lenses, an object-glass, and 

 an eye-glass, the foci 

 of which are in the 

 "same place. Here is 

 a diagram (Fig. 40) 

 that will explain it. 

 Let A B represent the 

 ravs from the moon 



falling upon the object glass c D, then the 

 imae formed upon this glass being seen 

 through the eye-glass E F, will have its 

 apparent diameter magnified accordingly. 

 The image will he inverted with respect 

 to the object, but this is not of any con- 

 sequence for the purposes to which it is 

 applied. Its power may be calculated 

 thus, if the object-glass have a power 

 = 20, and that of the eye-glass = 8, the 

 object will be magnified to 20x8 = 160 

 times. 



141. T. Describe the camera obscura ? 



P. It consists of a rectangular box, 



ABCD,with 



^ U^y-^i- a circular 



f """- -^ 'A$W a P erture in 



ft "TH T^K/: ^ont, fitted 



with a 



c rooveable 

 Fig. 41. tube T, hav- 



ing a convex lens at its extremity. In 

 the bodv of the box is a mirror, E I n D, 



Fig. 40. 



placed at an angle of 45, and at the 

 upper and back part a plate of ground 

 glass, E F G D. The rays of light pro- 

 ceeding from the object Q P, pass through 

 the lens, form an inverted image in its 

 posterior focus, which is received upon the 

 mirror, and is reflected upon the ground 

 glass. 



142. T. What is the camera lucida? 



P. It is an instrument similar to the 

 camera obscura in its effects, but smaller, 

 and therefore used chiefly lor drawing 

 distant objects. It consists of a quad- 

 rangular glass prism, by which the rays 

 from an object are twice reflected, and 

 thus form an image on a sheet of paper 

 placed underneath it. As the prism re- 

 volves upon an axis, it can be turned to 

 the proper position. 



[Grandfather Whitelimd docs not consider it 

 mri'hsiiry to describe the imcmscopt s, magic-lan- 

 terns, &c.. as they are only modifications of the 

 same principles as tho.se already hud dcwn.J 



