DIFFKACTION RAYS AEE IMAGE-FORMING 



59 



have a dry object-glass of nearly 180 angular aperture. This is readily 

 seen by fig. 45. By the arrangement presented in the figure the cover- 

 glass is practically the 

 first surface 

 jective, for 

 lens, 

 fluid, 



FRONT LENS 



the 

 and 



glass 



geneous. 



OBJECT IN AIR 

 SLIDE. 



IMMERSION FLUID 

 COVER CLASS 



FIG. 45. Diagram illustrating difference of emerging 

 pencil without and with balsam. 



of the ob- 

 the front 

 immersion 

 the cover - 

 are nil homo- 

 and of the 

 same refractive index, 

 and consequently they 

 form a front lens of 

 extra thickness. When 

 the object is close to 

 the cover-glass the pencil radiating from it will be very nearly 180. 

 and the emergent pencil will be seen to utilise so much of the back 

 lens of the combination as is equal to twice the focal length of the 

 objective, as shown in the Inner circle of fig. 46. 



If now we run Canada balsam beneath the cover-glass so as to 

 immerse the object, the pencil taken up by the objective is no longer 

 180, but only 122 ; but in spite of that the diameter 

 of the emergent pencil is lanjer than it, was when the 

 angle of the pencil was 180 in air, and is represented 

 by the outer circle in fig. 46. In both these cases 

 the power is identical ; it follows, therefore, that the 

 greater diameter of the emergent pencil from the 

 back of the combination denotes the greater aperture. 

 of the immersion objective over that of the dry one, 

 although it possessed an angle of 180. From this escape is impos- 

 sible, and it is for this reason that opticians make the back lenses of 

 their immersion object-glasses larger than those of dry ones of the 

 same power. 



Many further illustrations might be given, but none affording 

 greater facility than the following, viz. : ' Select a good specimen of 

 Amphi pleura, pdluclda and use oblique illumi- 

 nation, bringing out clearly the striation. 



' On removing the eye-piece, placing the 

 pupil 011 the air-image of the diatom, and 

 looking down on the lens, the direct incident 

 beam will be seen emerging as a bright spot. 

 and exactly opposite and close to the iar<jin 

 a, faint bluish light (see fig. 47). If now a 

 small piece of paper is placed on the back lens 

 of the objective so as to just cover up the blue 

 light, and the eye-piece is replaced, the diatom 

 is still visible, but all the striation which was 

 imaged by the blue marginal light has entirely 

 disappeared. The latter must therefore consist 

 of image-forming rav>.' 



Enough has thus been advanced to enable the student of even 

 the elementary principles of modern object-glass construction to 



FIG. 46. 



FIG. 47. Back of lens 

 on removing eye- 

 piece when A.pellu- 

 cida has been resol- 

 ved, showing spot of 

 bright light and faint 

 bluish spot opposite. 



