REFLEXION OF LIGHT BY SMALL SPHEEES, ETC. 



23T 



limitation, that the pencils emerging from the source of light, as a, 

 glance at the figure shows, always pass through the corresponding 

 marginal portion only of the objective, so that, for a point b of the 

 margin of the sphere, situated to the right, the left half of the 

 objective is ineffective. Under these conditions an elevation of 

 the luminous point 1) will act as a displacement of it towards the 

 left, i.e. the inclination of the pencil reaching the objective, or of 



its axis, is increased by the 



elevation, but that of the 



refracted one diminished 



(Fig. 130). The image-point 



V therefore moves to the 



right, and for the like reason 



a moves just as much to the 



left. The image of the 



luminous margin therefore 



becomes successively smaller ; 



the bright ring contracts more 



and more, until at length, if 



the elevation is continued, it 



will appear only as a luminous 



point occupying the middle 



of the dark shadow-image of 



the sphere. It is assumed 



that the image-points a' and 



I) ', notwithstanding this dis- 

 placement, do not essentially 



alter their original distances 



from the eye-lens, otherwise a distinct image 



would not be perceptible. In most objectives 



the image vanishes so quickly, that the last 



stage, the bright point in the centre, is clearly 



observed only with extremely small spheres, 



that is, those which require only a very 



slight displacement. This is the case, for 

 instance, with the higher powers of Oberha3user and Hartnack, with 

 the No. 7 of Beneche and Wasserlein, &c., whilst the No. 9 of the 

 last-named optician shows, when small spheres of 25 mic. in 

 diameter are used, not only the central point, but on still lower 

 focal adjustment (when the image-points alter their relative 



FIG. 130. 



FIG. 129. 



