MEANS OF DIVIDING THE PENCILS OF RAYS. 



49 



we must not pass over in silence the principles on which their 

 construction is based. 



If we disregard the earlier attempts of this kind, the multiplica- 

 tion of the image is effected in an essentially identical manner, 

 namely, by the splitting up of the pencils of rays usually 

 immediately after their passage through the objective into two 

 or more equal parts, each of which is directed through a separate 

 tube to the corresponding eye-piece. This splitting up is effected 

 either dioptrically or katoptrically ; dioptrically through an achro- 

 matic prism, as shown in Fig. 14, katoptrically by combinations 

 of prisms, which divert the pencils in different directions by 

 total reflexion (Figs. 15 and 16). The optical action of these 



FIG. 14. 



FIG. 15. 



FIG. 16. 



prisms in dividing the pencils into two parts is evident from the 

 construction ; it is also obvious that if in Fig. 16 the upper prisms 

 (or the faces at which the second reflexion takes place) are 

 turned round 90 a complete reinversion of the objective-image 

 must be caused. The optician has it therefore in his power to 

 give an erect or an inverted position to the final virtual image. 

 If a division of the cones of light into three or four pencils is 

 to be effected, the refracting or reflecting surfaces, which produce 

 the division, must be inclined to the median line in the same 

 number of directions. The edges in Figs. 14 and 15 will therefore 

 become solid angles with three or four limiting faces ; the middle 

 prism in Fig. 16 will become a three- or four-sided pyramid. The 

 same effect would be attained by repeating the splitting-up of 



E 



