38 GORHAM, ON THE MAGNIFYING 



The magnitude of the image is of course clue to the proxi- 

 mity of the object to the visual organ, the rounded appearance 

 of its ends to the circular form of the pupillary aperture, while 

 the dark bands are produced by interference. These pheno- 

 mena claim a more attentive examination. 



That the apparent magnitude of the luminous space is so 

 increased that the latter loses its linear form, and becomes a 

 plane, is only another example indeed of the general law in 

 optics, that all bodies, without exception, appear to grow 

 larger as they approach the eye, and to diminish as they 

 recede from it. But here an objection may be naturally raised 

 by one who has not familiarised himself with such inquiries, 

 or with the refracting powers of the eye. He finds from direct 

 observation, opportunities for which occur daily, that remote 

 objects do appear diminished in accordance with the law to 

 which we have referred, and with respect to objects at such 

 distances, he is inclined therefore to acquiesce in its correct- 

 ness. But on holding a small body, a needle we will suppose, 

 close to the eye, he is disappointed on discovering not only 

 that it is not magnified, but that it is altogether invisible. 

 Such an experiment has doubtless been performed by many, 

 and from its failure it has been concluded, and not without an 

 appearance of reason, that the body was held too near to the 

 eye to be visible, which however is not the case, as we have 

 endeavoured to show in a former experiment. But this very 

 failure indicates the necessity of means to an end. For if 

 having satisfied ourselves theoretically that the eye is endowed 

 with certain capabilities, which we have reason to believe 

 there is a possibility of developing ; and if, on the application 

 of certain known laws in optics, some definite figure which it 

 was anticipated should certainly result, does not make its 

 appearance, we are driven to the conclusion, that the failure is 

 attributable to the experiment itself. A fresh trial, however, 

 is perhaps crowned with success, and it is thus that we become 

 possessed of new optical instruments, the value of which is 

 directly proportionate to the importance of the laws they are 

 designed to illustrate. For what are all optical instruments, 



The glass or tin-foil should now be mounted on a piece of cardboard of 

 the required dimensions to fit the diascope, and having a hole about one 

 quarter of an inch in diameter punched from its centre. For this purpose 

 the thin tracing paper used hy architects is the best, as it answers the 

 double purpose of keeping the glass in its place, and preventing too much 

 light passing through the apertures. 



The dimensions of these apertures should be about the l-15th of an 

 inch by the l-135th of an inch, or nine times as long as broad (9 : 1 

 • : Ti! • rk)- These dimensions can be easily ascertained by a micrometer 

 with the aid of a microscope. 



