INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



777 



tlie front lens, the middle one is the image of the diaphragm at the 

 back of the objective ; or, if this diajihragm is sufficiently large, the 

 margin of the j^osterior system ; the inner circle is the image of the 

 end of the tube h, and within the area of this will be an inverted 

 picture of external objects crossed at the centre by the lines on the 

 glass; the objective c and the eye-lens / forming a sort of minia- 

 ture telescoj)e, and having the lines as a common focal point : the 

 smaller cii-cle would disappear if the end of the tube b was large 

 enough, and there would be but these two — the periphery of the front 

 lens, and the image of the diaphragm or posterior system —and it is 

 with this last we are to deal. A piece of tissue paper, or cap with 



Fig. 8. 



Fig. 



Fig. 10. 



Fig. 11. 



ground glass, is now put on at b, and immediately a soft light fills 

 the field, and the lines appear like cobwebs stretching across it. 

 The sector arm carrying the lens / is now swung round until the 

 intersection of the lines is tangent to the image of the margin of the 

 posterior system or diaphragm, as in Figs. 9 and 10, which represent 

 the circles as they would appear with very small angles ; with wide 

 angles they are foreshortened as in Fig. 11, where the larger circle is, 



Fig. Vi 



Fig. 12. 



Fig. 14. 



as before, the margin of the front lens ; the next inner one is the 

 image of the diaphragm, and the smaller (partly obscure) is the 

 bright field still visible, and which gives the exaggerated angle to 

 measurements made in the old way. The sector arm may be swung 

 many degrees farther on each side before this will disappear. When 

 the fine lines are thus projected on the face of the front lens, they 

 will, as in Fig. 12, mark the extremities of a diameter of the circle 

 which, if stopped out, would exclude all light from passing through 



