Dr. Wollaston^s Microscopic Doublet. 269 



with an oblique illumination in a certain state of intensity; even 

 the easiest of the class are very ill brought out with direct light, 

 which, if condensed upon them, seems to drown and confuse 

 the more minute markings altogether. I am here speaking of 

 them as transparent objects, but the easier ones are also visible 

 as opaque bodies, provided they are illuminated on the same 

 principle. Now Dr. WoUaston has selected precisely that kind of 

 illumination for them which is least favourable to delicate per- 

 spicuity, by condensing the direct light of a diaphragm of three- 

 tenths of an inch aperture, into the space of one-twentieth of an 

 inch, which of course renders it far more intense than day-light. 

 I can only say, for my own part, that I have not the grace to 

 see the lines on the poduray hrassica, and other objects of equal 

 difficulty, even with a one-sixtieth of an inch lens without 

 oblique light. 



Notwithstanding what I have said concerning Dr. WoUas- 

 ton's illuminative apparatus, I shall now shew that it may, by 

 proper management, be made to hide aberration, shew off 

 bad object-glasses to advantage, and to yield a most useful and 

 effective illumination, (when used with bright day -light,) af- 

 fording easy and certain means of developing the tissue of the 

 lined objects, and improving the distinctness of vision in almost 

 all others. All that is necessary for this purpose is merely to 

 cause it to act with diverging rays ^ instead of converging 

 ones, and to make the diaphragm eccentric -j- instead of centrical, 

 at least for the lined objects, (a mode of using it exactly oppo- 

 site to that recommended by Dr. Wollaston,) without changing 

 any thing in the construction. I shall hereafter suggest a mo- 

 dification of the Doctor's apparatus, which precludes the 

 necessity of employing a long tube, and is therefore more 

 convenient^ though I do not pretend that it performs a whit better 

 than his ; my improvement will adapt itself readily to any mi- 

 croscope or engyscope. 



Lambeth, Feb. \, 1830. 



• Br remwing the illuminating lens to a sufficient distance from the object, to 

 allow the rays to cross and diverge before they arrive at the object ; this distance, 

 and with it the strength of the light, must be varied to suit the power, object, and 

 state of the atmosphere : the apparatus is, I think, useless with aitilicial hght. 



I This gives oblique light in the direction of a line drawn from the centre of 

 the visual pencil, to the centre of the image of Uw diaphragm in it. 



APRIL— JUNK, 1830. ''" • '^ ^ 



