18 SELECTION AND USE 



When we examine an object, first at a distance, and then 

 close at hand, we see it through the medium of two different 

 sets of rays, those in the latter case entering the eye in such a 

 direction that the image thrown on the retina is larger than the 

 image produced when the object is more distant. The lens acts, 

 however, by bending the rays so that the same set, which, if 

 allowed to pursue their natural direction would not produce a 

 distinct image, are caused to enter the eye in such a direction that 

 the image is large and clear. The manner in which the lens acts 

 to produce these effects is not difficult to understand. It is true 

 that the ultimate causes which produce these phenomena are 

 beyond our knowledge, but in this respect the ablest philos- 

 opher has very little advantage over the veriest tyro. It may 

 be difficult also for the general reader to follow the mathe- 

 matical demonstrations of the action of lenses. There are, 

 however, a few simple facts which are easily understood, or at 

 least demonstrated and accepted as facts, and which, when 

 clearly and firmly grasped by the mind, render the construction 

 of the microscope comparatively easy of comprehension. 



There are two ways in which the subject may be studied. 

 We may examine the facts experimentally, by using lenses 

 and actual eyes in the way we have described, or we may fol- 

 low the course of the rays as laid down in any good book on 

 optics. A combination of both methods will of course give 

 the clearest views on the subject, and we would therefore ad- 

 vise the reader to provide himself with a few lenses of various 

 degrees of curvature, and consequently of various magnifying 

 powers, and test all the statements made in the text. He will 

 thus acquire such a practical knowledge of the action of lenses 

 as can be obtained in no other way. For this purpose the 

 cheapest lenses are good enough. One or two cheap magni- 

 fiers and a few glasses from old spectacles will serve every pur- 

 pose. The simplest methods of arranging such lenses will be 

 found in a note on a subsequent page, and although very ac- 

 curately made tools are required for the construction of ser- 

 viceable optical instruments, it will be found that a very 

 large number of simple but valuable experiments may be 

 worked out with the aid of a few wooden rollers and a little 

 paper and paste. 



