LIGHT AND SOUND. 



gives inverted images of objects, it is well enough 

 adapted for celestial observations, in which this 

 inversion is of no moment, but not at all for 

 terrestrial observations. But by introducing a 

 convex lens, or more frequently a combination of 

 convex lenses, to produce an inverted image of the 

 first image, and then magnifying this by a suitable 

 eye-piece, the terrestrial telescope is produced, 

 which is called an achromatic one, if the object 

 lens be achromatic. 



The reflecting telescope differs from the refract- 

 ing, primarily, in having the image formed by 

 reflection from a concave mirror ; and secondarily, 

 in requiring an arrangement for throwing the 

 image out of the axis of the telescope, as the 

 head of the observer, being on the same side of 

 the mirror as the object, would obstruct the light. 

 In the Newtonian reflector, a small plane mirror 

 is placed in the axis of the telescope, at an angle 

 of 45 with it, and rather nearer the mirror than 

 its principal focus. The rays from a distant 

 object, only a small number of which are ob- 

 structed by the plane mirror, are reflected by the 

 large mirror, so that they would form an image 

 in its principal focus, but striking on the small 

 mirror, they are turned through a right angle, and 

 the image is formed at the side of the tube. The 

 tube is perforated at this place, and a small tube 

 carrying the eye-glass is inserted at right angles 

 to the large tube. As each reflection is attended 

 by a partial loss of light, Sir William Herschel 

 dispensed with the small mirror, by slightly tilt- 

 ing the large mirror, as at mn in the diagram, 



Fig. 28. 



and thus throwing the image to the side of 

 the tube at a, where the eye-tube is placed. 

 In both forms of the reflecting telescope, the 

 magnifying power and the diameter of the field 

 of view are calculated exactly as in the case 



of the refractor ; but for all telescopes, they may 

 be found experimentally, and with sufficient accu- 

 racy, as follows.* Look with one eye through the 

 telescope at an equally divided scale, placed at a 

 distance (a brick-wall will generally suffice), and 

 count the greatest number of divisions visible; 

 then, with the other eye looking along the tele- 

 scope, count the number of divisions in the same 

 length : the number of times this latter number 

 is contained in the former is the magnifying power 

 in diameters. For the field of view, select from 

 an atlas of the stars, or a celestial globe, a star 

 which is on the equator, or nearly so ; and direct- 

 ing the telescope to it, find how many seconds it 

 takes to traverse the widest part of the field ; then 

 each second of time will correspond to 1 5 seconds 

 of arc. For example, if the star remains visible 

 for 60 seconds, the diameter of the field of view is 

 goo seconds of arc, or quarter of a degree. 



THE MICROSCOPE. 



The microscope, or instrument for viewing small 

 objects, may be either simple or compound. The 

 simple microscope consists of a single convex lens, 

 of short focal length, which magnifies an object 

 by enabling the eye to view it at a less distance 

 than it could do without the lens ; and its magni- 

 fying power is the number of times its focal 

 length is contained in the least distance of dis- 

 tinct vision, which is, on the average, about ten 

 inches. Thus, a quarter-inch lens will magnify 

 about 40 diameters. The compound microscope 

 consists essentially of an object lens, or combina- 

 tion of lenses to form a magnified image of the 

 small object, and an eye-glass to magnify this 

 image, as in the telescope. An additional lens, 

 called the field-glass, is, however, always intro- 

 duced between the other two, for a reason that 

 will now be explained. Let Pq be the minute 

 object placed in front of the achromatic com- 

 bination, A, which acts like a single lens. The 

 pencil of rays diverging from p would be brought 

 to a focus at p', the line pp' passing through 

 the centre of A ; but the field-glass, F, being 

 interposed, the pencil is brought more quickly 



Fig. 29. 



to a focus at P. Similarly, the pencil from 

 q is brought to a focus at Q instead of g', and 

 an inverted image, PQ, is formed of pq. The 

 image PQ being in the focus of the eye-glass 

 E, a pencil from any point of it emerges from E 

 parallel to the line joining the point with E, and 

 the eye sees a highly magnified image in this last 

 direction. The use of the field-glass is thus to 

 cause the pencils, by which each point of the 



image is formed, to converge to the axis of the 

 microscope, so that a more manageable eye-piece 

 can be used. The magnifying power of the micro- 

 scope is that of the combination, A, multiplied by 

 that of the eye-glass, E, if we neglect the small 

 diminution caused by the field-glass. If the 

 distance, A/', be 12 inches, and A/> the sixth of 

 an inch, the image p'g' is 72 times the size of qp. 

 And if the magnifying power of E be 10, the 



251 



