164? On Distinctness of Vision by the Polarizing Microscope. 



of the admitted light as can be done by simple means, to a 

 focus in the same place as the object to be examined. For 

 this purpose I have used with success a plane minor to direct 

 the light, and a plano-convex lefts to collect it." In describing 

 the apparatus itself, he enjoins that this " plano-convex lens, 

 or one properly crossed, so as to have the least aberration, 

 should be about three-quarters of an inch focus, having its plane 

 side next the object to be viewed ; and at the bottom is a cir- 

 cular perforation A, of about three-tenths of an inch in diame- 

 ter, for limiting the light reflected from the plane mirror, and 

 which is to be brought to a focus at a, giving a neat image of the 

 perforation A at the distance of about eight-tenths of an inch 

 from the lens ET (the plano-convex lens) and in the same 

 plane as the object which is to be examined. * * * * The 

 lens ET, or the perforation A, should have an adjustment by 

 which the distance bet-ween them may be varied, and the image 

 of the peif oration be thus brought into the same plane with the 

 object to be examined." * * * " For the perfect performance 

 of this microscope," Dr. Wollaston adds, " it is necessary that 

 the axes of the lenses and the centre of the perforation should 

 be in the same right line. This may be known by the image of 

 the perforation being illuminated throughout its whole extent, 

 and having its 'whole circumference equally 'well-defined. For 

 illumination at night, a commonhnWs-eye lanthorn viay be used 

 'with great advantage." 



In the Appendix to this paper, Dr. Wollaston gives the 

 following directions for the adjustment of his illuminating lens. 

 " The position of the lens may be varied so as to bring the 

 image* of the perforation into the same plane with the object to 

 be viewed. * * * * Supposing the plano-convex lens (the 

 illuminating lens) to be placed at its proper distance from the 

 stage, the image of the perforation may be readily brought into 

 the same plane with the object, by fixing temporarily a small 

 'wire across the perforation with a bit of wax, viewing any ob- 

 ject placed upon a piece of glass upon the stage of the micro- 

 scope, and varying the distance of the perforation from the 

 lens by screwing its tube until the image of the 'wire is seen 

 distinctly at the same time 'with the object upon the piece of glass." 



From these extracts we do not say it is manifest, but we say 

 it is demonstrable, that Dr. Wollaston was not in the least 

 degree acquainted with the method which I subsequently pub- 

 lished, of illuminating microscopic objects, and which is now in 



* That is, tlie luell-dcfined circular image in the conjugate ybc«« of the 

 perforation considered as a circular object, from which the rays diverge, — 

 not certainly in the conjugate focus of the rays which pass through the 

 perforation. — D. B. 



