464 ON PHTSIOLOGICIL LIMITS OF MICROSCOPIC VISION. 



place, e.g., microscopic intervals between ruled lines together with 

 the lines themselves must be rendered visible by contrast of light 

 and shade, as well as by equalisation of their scale of size with the 

 dimensions of the retinal percipient elements. Hence the necessary 

 relation between magnifying power and aperture of a microscope 

 objective. But supposing in the next place that there should arise 

 with the fulfilment of these conditions a new difficulty — and the 

 case does so arise— that, namely, of diffraction, are we to interpret 

 it as a failure of the microscope or of the eye ? In considering this 

 question the following facts must be kept in mind : — 1. There are 

 two distinct sources of diffraction. When points in an ohject become 

 self-luminous by the diffraction occasioned in the ohject, so much 

 detail is added to the picture by a wide angled objective which 

 takes in such diffracted rays and re-produces by its focussing 

 function the image of the diffracting particles. Such diffraction is, 

 therefore, a source of gain for which the microscope is to be 

 credited. 2. The second diffraction effect has been already ex- 

 plained as the necessary consequence of the reduction of the 

 microscope image into a bundle of bright pencils entering through 

 the pupil of the eye, and as has also been explained, the evil 

 results of this condition are attributable to excessive angular 

 amplification, but not to faulty lens-construction. The eye is not 

 constructed to work with such abnormal conditions, and vision is, 

 therefore, impaired physiologically. 3. As a matter of fact the 

 modern microscDpe lens-combinations work so well that undulations 

 of shorter wave length than those which excite sensations of light 

 define (without shewing " interference ") finer lines and interspaces 

 on a photograph surface than have ever been seen by the eye, 

 looking at the same object through the same lens-system and under 

 the same illumination. That is to say the retina is not so sensitive 

 as photograph paper, and the dioptric media of the eye cannot 

 transfer the microscope image so perfectly as it can be thrown 

 directly on the chemically prepared paper through the microscope. 



Thus then it is plain that resolution of minute objects is not 

 limited by characters of physical light but by physiological in- 



