ON THE MOUNTING OF EADUL.E FOR MICROSCOPIC EXAMINATION. 

 By the Kev. E. W. Bowell, M.A. 



Bead 12th March, 1915. 



PLATE VII. 



It is usual to mount objects destined for microscopical examination in 

 some medium having approximately the same refractive index as the 

 glass (or the average of the glasses) used in the construction of the 

 lenses of the instrument. When this rule is transgressed the full 

 resolving power of the microscope cannot be utilized. Eadula^ are, 

 in practice, usually mounted in a medium of lower I'efractive index 

 than glass. This is done because otherwise they would be invisible, 

 or nearly so, unless viewed by polarized light. 



The polariscope method is very satisfactory with large species and 

 low magnifications. The object is mounted in Canada balsam, and is 

 barely visible on the slide, but with the aid of polarizer and analyser 

 it shows up brilliantly. When, however, the smaller forms are 

 reached, it is found that little can be seen, unless prisms of 

 phenomenal size and transparency are employed, together with 

 a powerful light. I therefore regard the polariscope method as 

 ineligible. 



Tlie ordinary preparations of raduloe in glycerin jelly are of 

 varying visibility, because it is not possible to standardize the 

 amount of water or glycerin included in the mount. I have 

 successfully used in place of glycerin jelly Professor Gilson's euparal, 

 an artificial resin of low refractive index. It is much easier to employ 

 than glycerin jelly, requires no ringing, and has the advantage of not 

 being an aqueous medium. But it has the optical disadvantages 

 inseparable from a medium of low refractive index. Dr. Boycott 

 uses Farrant's medium, by which results similar to those of glycerin 

 jelly are produced. Oxidation eventually occurs. 



Glycerin jelly has been advocated and used by practically all the 

 highest authorities on the subject of raduloe. Therefore I have 

 returned, time after time, to its use ; only to be convinced each time 

 that the optical disadvantages involved were real and insuperable. 

 The camera is a severer critic of the microscopic image than the most 

 fastidious microscopist, because it possesses no power of accommodation, 

 and is incapable of ignoring distortion of form. And the camera, as 

 I read its verdict, is plainly adverse to mounting in any but 

 a homogeneous medium. Particularly bad are the results with high 

 powers, for several reasons. The radulae consist of fine serrated lines 

 of structure, and these of themselves form diffraction gratings 

 interfering with the normal diffraction system of the instrument. 

 The apparent distance between the two layers of structure is greatly 

 exaggerated, in addition to the exaggeration which naturally results 

 from the use of a high numerical aperture, which is necessary for 

 definition. And that aperture itself requires to be cut down in order 

 to produce contrast, so that the efficiency of any objective is reduced 



