24 On the Mounting of the Biatom-prism. 



in fractions of an inch is '044, radius "OSIS, diameter "062, glass 

 cover • 007, distance of focus from front • 004, as shown at a point 

 in the glass by the dotted hues. The thickness of the cover and 

 lens together is '051. In Fig. 3 the radius and diameter is the 

 same as the dry lens — Thickness of lens ' 034, ditto of water film 

 "012, ditto glass cover '007, total combined thickness "053; so 

 that the corrected thickness of the water front exceeds that of the 

 dry lens and cover by • 002, or s^th part of an inch. This differ- 

 ence may be accounted for by the less refractive power of the 

 water. 



The effect of this immersion lens is to give greater clearness and 

 brilliancy to the object, and render markings more distinct that were 

 before scarcely visible with the dry lens. This is in fact attributable 

 to the saving of hght and comparative absence of refraction and 

 reflexion from the top surface of cover and front of lens. But the 

 great merit consists in the perfect correction that the adjustable 

 thickness of the water stratum affords in compensating for every 

 thickness of cover. Nor is the thickness of an immersion front a 

 matter of particular nicety, for it can be made as thin as desirable ; 

 the water will occupy the place of the deficiency. 



In the three diagrams I have accurately transferred all the 

 dimensions, and it can at once be seen how a thickness of front en- 

 sures both correction and large angular aperture, by spreading out 

 the cone of rays in its passage through the lens, after the first 

 refraction, so that the marginal ones fall on the edge of the convex 

 back, and then emerge in a direction suitable for entering the middle 

 and posterior combinations, conveying the full angular pencil 

 through the series. 



I have not yet ascertained whether the " aplanatic searcher " has 

 any influence in improving definition, as no description of its con- 

 struction has yet appeared, and it exists more in name than in use. 



V. — On the Mounting of the Diatom-prism. 



By F. W. Griffin, Ph.D. 



The unilateral system of illuminating transparent objects, intro- 

 duced by the Kev. J. B. Eeade, F.E.S., bids fair to be more and more 

 generally employed as its important advantages become more widely 

 known. But there are practical difficulties in fitting the diatom- 

 prism to any large and complete stand, since to obtain the most 

 oblique beam it has to be brought almost close under the slide, 

 while elaborate traversing and rotating movements require a thick- 

 ness of stage which keeps it too far ofi" for its full effect. In the 

 Eoss model this thickness is certainly reduced to a minimum in the 



