LIMITS OF OPTICAL CAPACITY OF THE MICROSCOPE. 409 



the rays by which these images are formed, help us to an 

 interpretation of the physical agencies at work, and shew us also 

 why the extreme amplifications employed render vision through 

 the microscope more imperfect than through any other optical 

 instrument, such as telescope, or camera. But the analysis of 

 these physical agencies and effects, involves the consideration of 

 the eye itself, as an optical instrument through which the micro- 

 scope image must pass to reach the perceiving organ. And apart 

 from the imperfections arising from aberrations and dispersions of 

 rays in the instrument, other imperfections of the retinal image 

 will be found in considering the more or less favorable conditions 

 under which the microscope image enters the eye. The area into 

 which the microscope image is collected at the eye spot (over the 

 ocular,) varies in size with the amplification, and is smaller in pro- 

 portion as the amplification is greater. And this variation of size is 

 accompanied by variation in brightness of image and distinctness of 

 detail. If the area of illuminated image entering the pupil is 

 smaller than that of the pupillary aperture, loss of brightness is 

 felt. For the condition of most effective illumination (brightness 

 of image) is that, which obtains when the area of image at the eye spot, 

 and the area of the pupil, are equal. On the other hand, a small and 

 intensely bright spot of light in front of the pupil presents the exact 

 condition under which entoptic shadows obscuring the image are 

 thrown with it on the retina. But as brightness of image is as 

 necessary to distinct vision as any mere amplification of detail can 

 be, it follows that a suitablejrelation of " aperture" to " magnifying 

 power" must be maintained in every good objective; for 

 "aperture" in this particular case means the measure of light 

 admitted with the image-forming rays ; and as a larger measure of 

 light is required in proportion to the increase of magnifying power, 

 30 it is only when these two factors are suitably proportioned that 

 details in the objective will be rendered clearly visible in its micro- 

 scope image. And again, as respects the bundle of rays collected 

 into a smaller or larger area at their entrance to the pupil, the 



regulation of illumination from without is better maintained with a 



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