322 NOTES AND MEMOKANDA. 



to a stem or tail-piece made so as to bo swung in this position, but 

 the invention which I consider to be novel and therefore desire to be 

 secured to me by the herein in part recited letters patent, is — 



" First. The making the stem A, which carries the sub-stage B, 

 to swing to the right or to the left either below or above the stage of 

 the microscope on a centre sleeve socket or joint I, the axis of revo- 

 lution whereof is the line X Y, in the plane of the object to be viewed 

 on the stage C, intersected by the optical axis, that is, the line N O, 

 passing through the centre of the body F and the object-glass of the 

 microscope, substantially as described and shown in the drawing. 



" Secondly. The arrangement herein described and shown in the 

 drawing for enabling the object-stage C to swivel or turn on a centre 

 or pivot within the sleeve or socket I, so that the axis of rotation of 

 the object-stage shall be from the same centre as that on which the 

 stem or part A turns to the right or left, and the method of clamping 

 the object-stage C in the required angle, as herein described and shown 

 in the drawings." 



"Penetration" of Wide-angled Objectives. — It has been ob- 

 jected to wide-angled lenses that they possess less penetrating power, 

 or, more jiroperly, less depth of focus than narrow-angled lenses ; that 

 is to say, that the layer of an object, that can be seen without change 

 of focus, is thinner with wide than with narrow-angled lenses. 



Dr. Blackham, the President of the Dunkirk (U.S.) Microscopical 

 Society, says that if this were true it would be an argument in favour 

 of the wide-angled lenses, instead of against them ; in reality, however, 

 it does not depend upon the aperture, but is only residual spherical 

 aberration, which can be left in and distributed in a wide-angled lens 

 as well as in a narrow-angled one. This will be readily api^reciated 

 upon considering the action of an uncorrected plano-convex lens of 

 crown glass. The rays from the nearer surface of the object which 

 impinge upon the peripheral portions of the lens would, if the lens 

 were free from spherical aberration, be brought to a focus further 

 back than those from the further surface of the object which impinge 

 upon the central portions of the lens. As it is, however, they are 

 brought to the same focus, by reason of the spherical' aberration. 

 Such a lens has a good deal of penetrating power, or depth of focus, 

 but its definition is not satisfactory. The same holds true of all 

 objectives possessed of penetrating power, whatever their angular 

 aperture. The only legitimate method of obtaining depth of focus 

 or " penetration " is by increasing the anterior conjugate focus or 

 frontal distance, so that the thickness of the layer that it is desired to 

 see on each side of the true focal plane may be relatively small. 

 Thus a 1-inch objective with an anterior focus of 'SIT of an inch 

 will bear amplification up to 400 diameters, and at that power might 

 properly show, with reasonable clearness, a layer of the object on 

 each side of the true focal plane much thicker than that which a one- 

 fifth with only -018 of an inch of anterior focus ought to show at the 

 same amplification. It is perhaps true that, by skilful management, 

 the residual sjiherical aberration can be so distributed, that several 

 planes of an object can be in view at once ; but this is always at the 



