16 CONSTRUCTION OF THE MICROSCOPE. 



But when the rays are oblique to the_ axis, as 5 I, s" I, 1 1, t" ?', the 

 rays s' c, t' c, which pass through the centre, c, of the lens, will suffer 

 refraction at each surface ; but as the two refractions are equal and in 

 opposite directions, the finally refracted rays ef } cf" will pass from 



s' c, t' c ; and it will be found that s I, s" I' will be refracted to a com- 

 mon point,/", in the direction of the central ray s'f", and 1 1, t" I' to the 

 point / When the lens is of glass and equally convex, the focal dis- 

 tance will be equal to the radius. As each ray carries with it the 

 image of the object from whence it proceeded, it follows, that if those 

 rays, after intersecting each other, and having formed an image at their 

 intersection, are again united by refraction or reflection, they will form 

 a new image, and that repeatedly, so long as their order is not dis- 

 turbed. It follows also, that when the progress of the luminous ray 

 is under consideration, we may look on the image as the object, and 

 the object as the image ; and consider the second image, as if it had 

 been produced by the first, as an object, and so on. This is one of 

 the principles involved in the adaptation of these lenses to magnifying 

 objects. It has been shown in fig 6, that if the point of light be situ- 

 ated above the line of the axis, the focus will then be below it, and vice 

 versa; but the surface of every luminous body may be regarded as 

 comprehending an infinite number of such points, from all of which a 

 pencil of light-rays proceeds, and is refracted according to the general 

 law ; so that a perfect but inverted image or picture of the object is 

 formed upon any surface placed in the focus, and adapted to receive 

 the rays. 



And if the object be placed at twice the distance of the principal 

 focus, the image being formed at an equal distance on the other side 



