ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 293 



nearly approached this limit, that any very important improvement in 

 the way of a further development is no longer to be anticipated. This 

 limit to all optical observation, in the direction of minuteness, can be 

 approximately defined by half the wave-length (in air) ; at least 

 microscopical observation cannot be applied to objects which are 

 smaller to a considerable extent than half the wave-lengths, although 

 the latter can be somewhat exceeded with immersion lenses. 



The measure of the details accessible to our vision is not an abso- 

 lute one, but is related to the wave-length of the light by which in 

 any particular case the image is formed. There is, therefore, a 

 certain latitude which can be utilized to some extent in favour of optical 

 perception. In observations with white light those rays predominate, in 

 the formation of the image visible to our eye, which show the greatest 

 intensity in the visible spectrum. The mean wave-length will there- 

 fore correspond with the bright green, and may be taken as 0*55 /x. 

 Somewhat smaller wave-lengths, those of the blue rays, allow of 

 effective observations with so-called monochromatic illumination, the 

 advantages of which for the recognition of the finest details have long 

 been known to the microscopist. 



Still more favourable are the conditions of image-formation with 

 photography, since in this case the wave-length of the violet rays, 

 which are the active ones, is 0*40 /a only. The performance of 

 objectives under otherwise similar circumstances extends, therefore, 

 perceptibly further with photography than with direct observation. 

 Not only does the photograph show finer details at the limit of the 

 resolving power than would be directly visible to the eye, but even 

 when the object is not at the extreme limits of resolution, but the 

 correctness of the image is yet more or less problematical, it gives a 

 greater guarantee of the truth of the representation than does the 

 ordinary image. Hence photo-micrography, in difficult examinations, 

 has a value not to be underrated. 



A further step may be taken in this direction by utilizing rays 

 which probably lie far beyond the limits of the visible spectrum in 

 the ultra-violet. If these images are not directly visible, it is possible 

 to imagine them made visible by means of fluorescent substances. 

 But for this the optician must have materials for the construction 

 of the objective which possess at least the transparency of quartz 

 for the ultra-violet rays, without those properties which now preclude 

 its application for such a purpose, and substances of similar trans- 

 parency must also be found for imbedding the object and for the 

 immersion fluid. 



This consideration shows to what extent we must quit the sure 

 ground of experience if from our present standpoint we reckon on a 

 fundamental improvement of microscopy. The result of such attempts 

 leaves no prospect in the main of the realization in the future of 

 hopes and wishes which rest on the notion of an ever-extending and 

 unlimited improvement in our optical instruments. Judging from 

 what lies within the horizon of our present knowledge, a limit is put 

 to the range of our eyes by the action of light itself, a limit which is 

 not to be overstepped with the tools given us by our present know- 



