THE ANGULAR APERTURE OF OBJECT-GLASSES. 47 



openino^ of 1-30" diameter, fixed to a small g^lass tube in an 

 adaptor, in the place of the achromatic condenser, I illumi- 

 nated the prepared valve of a Pleurosigma Balticum, mounted 

 dry, by as dii-ect and straight a light as could be done ; and 

 under an object-glass (i), whose aperture I had reduced to 50°, 

 both sets of stria? were visible. I next increased the aperture, 

 by substituting a larger stop, to 65°, and the markings became 

 much more distinct. A similar result was obtained by suc- 

 cessively increasing the aperture to 75°, and, lasdy, to 90°, 

 when the distinctness of the markings was most strikingly 

 increased, and the whole object more brightly illuminated. 

 It cannot be doubted that a similar result would have been 

 obtained, had I been able still to increase the aperture of the 

 object-glass ; and if the author's theory was correct, in doing 

 so, the light being straight, the markings ought to have be- 

 come fainter and fainter, and disappeared entirely at last, as, 

 with every degree added to the aperture, fewer rays could 

 become refracted out of the field of the microscope. 



This experiment proves further, the light being direct and 

 straight, that the obliquity of the emerging rays inust be due 

 to the peculiar structure of the markings, and does not arise 

 from a difference of density, as assumed by the author ; and 

 further, that the visibility of the markings depends upon 

 aperture and not upon illumination, though the latter may 

 serve to increase their distinctness, while, without the former, 

 any kind of illumination would remain ineffective. 



A similar experiment, previously made, having made it 

 probable that the set of rays corresponding to the depressions 

 did not pass through them at all, but was completely inter- 

 cepted, and either refracted or reflected into the substance of 

 the valve towards the margin of the depressions, thus leaving 

 the latter themselves dark, it was desirable to devise another 

 experiment, on such a scale as would admit of a practical 

 proof regarding the phenomena concerned. For this purpose 

 I put a thin layer of Canada Balsam, nearly deprived of its 

 turpentine, so that it hardens as quickly as it cools, on an 

 ordinary glass slide, and, with the delicate bristles of a seed 

 of an Erodinae, I made a number of minute markings respect- 

 ing depressions while the balsam was yet soft, but not so soft 

 as to stick.* It being admitted that the markings on the 

 gyiosigma, for instance, consist of depressions in the siliceous 

 substance of the valve, and Canada Balsam having almost 

 the same refractive index as silica, agreeably to Mr. Wenham's 



* If the balsam is already too hard, it cracks, the surface of the depres- 

 sions, becomes uneven, and forms new sources of refraction, which is also 

 the case if the markings penetrate down to the glass. 



