310 



•WELLS'S NATURAL PHILOSOPHY. 



Fig. 2-4G. 



as though they proceeded from an ob- 

 ject behind the mirror, as at h. To an 

 eye at b also, the image will appear 

 larger than the object a, since the anglo 

 of vision is larger. 



If the rays proceed from a distant body, 

 as at E D., Fig. 247, beyond the cen- 

 ter, C, of a spherical concave mirror, A B, 

 they will, after reflection, be converged to 

 a focus in front of the mirror, and some- 

 what nearer to the center, C, than the prin- 

 upon any 



Fig. 24T. 



cipal focus, and there paint 

 substance placed to receive it, an im- 

 age inverted, and smaller than the object ; 

 this image will be very bright, as all the 

 light incident upon the mirror will be gath- 

 ered into a small space. As the object 

 approaches the mirror, the imago recedes 

 from it and approaches C ; and when situ- 

 ated at C, the center of the curve of the mirror, the image will be reflected 

 as large as the object : when it is at any point between C and / supposing / 

 to be the focus for parallel rays, it will be reflected, enlarged, and more dis- 

 tant from the mirror than the object, this distance increasing, until the ob- 

 ject arrives at/, and then the image becomes infinite, the rays being reflected 

 parallel* 



668. When an object is further from the 

 surface of a concave mirror than its principal 

 focus, the image will appear inverted ; but 

 when the object is between the mirror and its 

 principal focus, the image will be upright, and 



increase in size in proportion as the object is placed nearer 



to the focus. 



The fact that images are formed at the foci of a concave mirror, and that by 

 Tarying the distance of objects before the surface of the mirror, we may vary 

 the position and size of the Jmages formed at such foci, was often taken ad- 

 vantage of in the middle ages to astonish and delude the ignorant. Thua, 

 the mirror and the object being concealed behind a curtain, or a partition, and 

 the object strongly illuminated, the ra^s from the object might be reflected 

 from the mirror in such a manner as tc ^>ass through an opening in the screen, 

 and come to a focus at some distance beyond, in the air. If a cloud of smoka 



* In all the cases referred to, of the reflection of light from concave mirrors, the aper- 

 ture or curvature of the mirror is presumed to be inconsiderable. If it be increased be- 

 yond a certain limit, the rays of light iacident upon it are modified la their reflectioa 

 from its BurfacD. 



When will the 



images reflect- 

 ed from a con- 

 cave mirror 

 appear invert- 

 ed, and when 

 erect f 



