BOWELL : MOUNTING OF EADUL^. 273 



to about half of the normal. Further — and this is the most serious 

 matter of all, since it is the real forms of objects that we require to 

 trace, and not their spurious images — the particular structures that 

 we have to deal with here are themselves capable of setting up definite 

 disturbances of the image, by reason of their rounded or cylindrical 

 shape. In a mass of low refractive index we have placed a number 

 of rows of lenses of irregular form. They do not fail to present 

 untrue appearances the moment that they are illuminated by any 

 other than parallel beams. Thus, for example, the centres of the cones 

 in a Vertigo necessarily appear black instead of white in a photograph 

 taken at the critical focus ; while if a false focus be substituted the 

 result is indeed a dream, but different from reality. Even optically 

 short spikes appear as rounded knobs when we have passed a certain 

 measure of magnitude, necessitating a certain enlargement of 

 numerical aperture to render the object distinctly visible. 



I have experimented with many media for mounting radulae — 

 practically, I believe, with all that are available. Not to trouble you 

 with a long catalogue of rejected methods, I may just mention that 

 media of higher refractive index have most of the disadvantages 

 already described, together with some special ones of their own. 



But all this can he definitely and entirely avoided. The image with 

 high powers can be made quite equal (except in the matter of depth) to 

 that afforded by objectives of greater focal length. The full aperture 

 and resolving power of the objectives can be employed. Eye- 

 straining can be entirely obviated. The preparations may be made 

 quite permanent, and very much more distinct in every outline, by 

 the adoption of a process of staining and mounting in a homogeneous 

 medium, such as is used in almost all professional scientific researches. 

 Our difficulty is then only to find a method by which the radula can be 

 stained in such a way that all its details are clearly visible. 



Chitin itself, as it exists in the snail, cannot be stained by any 

 known method. But it is by no means difficult to effect a modification 

 of its chemical constitution so that it becomes coloured by intrinsic or 

 extrinsic matter. The chitin of all radulae — even the Vertigos — 

 contains enough iron to give a Prussian blue tint when it has been 

 partially or entirely separated from its organic connexion. Anything 

 that promotes oxidation will in a long or short time turn the radula 

 brown — the colour of the familiar iron-mould. This is very 

 frequently to be observed in old specimens which have been preserved 

 for many years in aqueous media. It denotes an excessive proportion 

 of water present in a glycerin jelly preparation. It is very usually 

 to be seen at the nascent end of a radula extracted by long boiling in 

 caustic alkali. Noticing one day that this brown colour at the end 

 was replaced in some of my radulae by a green verging upon blue, 

 I instituted inquiries and found that the tube used for washing them 

 had been previously used for containing a solution of the red prussiate 

 of potash. Experiments showed that not only green but strong blue 

 coloration could easily be obtained upon yellow radulae by the 

 application of this familiar reagent. Strange to say, styrax 

 preparations were found to have oxidized the radulae to a considerable 



