Royal Society. 301 



are capable. The best relative proportion of the foci of the two 

 lenses appears, from the trials made by the author, to be that of 

 three to one. The distances between their plane surfaces should in 

 general be about l*^ of the shorter focus, but should be varied by- 

 trial till the utmost possible degree of distinctness has been attained. 

 The lenses must be fixed in their cells with their plane sides next to 

 the object to be viewed. The exterior cell of the compound mag- 

 nifier should be formed with a flanch, so that it may rest upon the 

 piece that receives it. The plano-convex lens by which the object 

 is illuminated is inclosed in a tube about six inches long, blackened 

 in the inside, and having a circular perforation below of about 

 three-tenths of an inch in diameter, for limiting the light reflected 

 from the plane mirror. The centre of this aperture must be in the 

 common axis of the lenses ; and the image of the perforation formed 

 by the large lens must be brought, by proper adjustment of the di- 

 stance of that lens, into the same plane as the object to be examined. 

 With a microscope so constructed, the author has seen the finest 

 striae and serratures upon the scales of the lepisma and podura, and 

 the scales upon a gnat's wing, with a degree of delicate perspicuity 

 not attainable with any other microscope he has tried. In con- 

 sequence of the plane surface of the lens being next to the object 

 viewed, the microscope of Dr. Wollaston possesses the important 

 advantage of having its action undisturbed by the contact of a fluid 

 under examination. 



A paper was also read, intitled, " On the Stability of Canoes," by 

 W. Walker Master, R.N. ; communicated by the President. 



The author having, in a former paper, endeavoured to show that 

 the longitudinal axis on which a ship rolls, by the force of the wind 

 on her sails, does not pass through the common centre of gravity, 

 but always coincides with the plane of flotation, proceeds, in the 

 present memoir, to the demonstration of his second proposition, 

 namely, that the stability of a floating body is a maximum when the 

 part immersed in the fluid is equal to half its magnitude; or, which 

 is the same thing, when its total weight is half that of the fluid which 

 it would displace by complete submersion. For this purpose he in- 

 vestigates the case of a canoe, supposed to have no stability in itself, 

 and connected by an outrigger with a balance-boat at a certain di- 

 stance; and shows that the power of such a boat in preventing the 

 oversetting of the canoe, by the action of a horizontal force applied 

 to the sails, is greatest when its weight is exactly the half of that of 

 an equal volume of the fluid. Boats with outriggers, he observes, 

 arc admirably adapted for velocity, for they are enabled to carry a 

 press of sail without ballast ; they displace little water, and they 

 move near the surface, where the resistance is less than at a greater 

 depth. The application of a ballast-boat by an outrigger has, how- 

 ever, the di^advantage of tending to turn the prow of the canoe 

 towards the wind ; an inconvenience which the experienced Indian 

 obviates by constructing his canoe with one side nearly a plane, so 

 that the oblique influence of the fluid on the prow is balanced by the 

 resistance of the boat ; and the flat side of the canoe being always 



turned 



