1056 SUMMARY OF CUREENT RESEARCHES RELATING TO 



question. Witli high powers, however, the interposed water would 

 seriously interfere with the corrections. The objectives are con- 

 structed to work with air, and if the rays have to pass through water 

 there is a considerable disturbance of their action both as regards 

 aplanatism and achromatism. The same result follows from capil- 

 larity, by the action of which the upper surface of the water is 

 distinctly curved. 



"We are obliged therefore to come to the conclusion that M. 

 D'Arsonval's idea, though a not uninteresting contribution to the 

 history of suggestions on the construction of the Microscope, cannot 

 be realized in practice. 



Another advantage claimed by the inventor was the power of 

 using thick cover-glasses ; also coloured solutions for monochromatic 

 light for photography. 



In this connection it may be interesting to note an idea which oc- 

 curred to Hooke,* in regard to the use of water between the lenses. 

 " I provided me a Tube of Brass . . . ; into the smaller end of this 

 I fixt with Wax a good piano convex Object Glass, with the convex 

 side towards the Object, and into the bigger end I fixt also with wax 

 a pretty large piano Convex Glass, with the convex side towards my 

 eye, then by means of the small hole by the side, I fill'd the inter- 

 mediate space between these two Glasses with very clear water, and 

 with a Screw stopped it in ; then putting on a Cell for the Eye, I could 

 perceive an Object more bright than I could when the intermediate 

 space was only fill'd with Air, but this, for other inconveniences, I 

 made but little use of." 



Direct Vision Microscopes.f — Mr. T. E. Amyot, observing that 

 many of the old faults and deficiencies of these instruments remain 

 uncorrected and unsupplied, describees the alterations which he has 

 made in one, which have rendered it " perfectly available for many 

 purposes for which it was previously inapplicable, and in fact," as far 

 as his own requirements go, " a very useful instead of a nearly 

 useless instrument." 



The faults of all the instruments of this class with which he is 

 acquainted are the following : — 



1, The object examined is rendered indistinct by the amount of 

 side light which falls upon it in its exposed position. 



2. The stage arrangements are so imperfect that it is impossible 

 to examine any but the central portion oj; the slide, or at best such a 

 portion as has been previously arranged for examination. 



To correct the first fault nothing more is required than 1/3 in. of 

 metal tube blackened internally, the size of, and projecting beyond, 

 the stage aperture ; this too would easily carry a polarizing prism or 

 a spot-lens if desired. 



To remedy the second fault (the instrument operated on being 

 Dr. Beale's Class Microscope) the bell-shaped end is removed and in 

 its place is fixed a brass cylinder, with a gap in front for tiie use of 



* Hooke, K., ' Micrographia,' 1C67, preface, 

 t Sci.-Gossip, 1885, pp. 201-2 (1 fig.). 



