356 SUMMARY OF CUREENT RESEARCHES RELATING TO 



The art is to strike a happy mean between the two. The following are 

 three of the lessons I have learnt : — 



1. Not to work with light too high up in the spectrnm. 



2. Not to form an opinion entirely by spectroscopic results. 



3. Not to imagine that one sci'een is sufficient. 



Colours must be seen, but the following descriptions of screens 

 which prolonged experiments have proved to be the best may be of 

 assistance. For daylight, a piece of peacock-green, worked down so that 

 it is not too deep in tint, is combined with a very light-blue glass not 

 deeper in tint than a rather pale lilac petal. For lamplight, a thicker 

 piece of peacock-green is combined with a l)lue glass, somewhat of the 

 tint of a blue flower {Centaurea cyanus) common in cornfields. 



The ideal screen for visual microscopical work is one which, filtering 

 out the too pronounced red, softens down, but does not entirely cut out, 

 the orange and yellow lights. Twenty-five years ago any screen which 

 did not pass certain spectroscopic tests, by absolutely cutting out all 

 lights longer than a definite wave-length, was rejected; now we know 

 better. The fact is that our heads were swelled by the " table of resolving 

 powers" published on the cover of the R.AI.S. Journal, where the three 

 selected lights had wave lengths of 5,269 for visual, line F 4,861 for 

 screen, and 4,000 for photography.* I altered the visual to Gilford's 

 maximum 5,607, and photography to 4,341 ; this last should be brought 

 still lower down the spectrum to the photographic maximum through 

 glass of 4,603, and the screen placed at least as low down as h, or 5,184) 

 if not lower. 



Spectrum. — A prism spectrum is better for this purpose than a 

 grating, for a prism forms only one spectrum, and all the light which 

 is dispersed goes into it ; on the other hand, a grating makes several 

 spectra, and, as only one of them can be used, much light is lost, but 

 the dispersion of a Rowland's 14,400-line grating between E and G lines 

 in a spectrum of the first order is more than double that of an ordinary 

 flint prism. 



C4) Photomicrography. 



Ernemann's Cinema Micro-apparatus for Production of Serial 

 Pictures of Living Micro-organisms.t — This apparatus is so designed 

 that it may be applied in either a horizontal or a vertical position. An 

 essential condition for the production of useful serial photographs is 

 quick working and a harmonious co-ordination of movements, for many 

 preparations are only available for brief periods or are damaged if 

 exposed to intense light-rays. This necessary rapidity of movement is 

 attained by a handle which enables the operator to swing the camera 

 into or out of action as required. When the apparatus has once been 

 correctly centred, so that the optical axis of the Microscope passes 

 through the middle of the film-image, the camera is swung aside, and 

 the object can then be suitably illuminated and adjusted. When the 

 operator thinks fit he swings the camera back into its old position and 



* See this Journal, 1885, p. 972 — where the photographic resolving limit is put 

 at 127,000 lines for N.A. l-Q! 

 t Special Catalogue. 



