44 DR. F. D'ALQUEN ON 



Mr. Wenham states, in one of his valuable papers, that the 

 markings on test objects become visible by a contrast of light ; 

 and the attention of the reader will at once be brought to the 

 point upon which the whole question hinges, when I add that 

 the gist of Dr. Griffith's paper is an attempt to show how this 

 contrast of light is produced, and why the markings can only 

 be seen under an object-glass of large angular aperture, and 

 not with one which is deficient in this respect, however great 

 its magnifying power may be. In answering these proposi- 

 tions the Doctor states, in substance : — The markings, those 

 on a valve of a Gyrosigma, for instance, being in reality de- 

 pressions, the light, on passing through them, suffers greater 

 refraction from the perpendicular than the set of rays corre- 

 sponding to the undepressed, tliicker, and therefore more 

 highly refractive portion of the valve, and we have thus two 

 sets of rays of different degrees of obliquity — the former of 

 which, as the most oblique, is tilted out of the field of the 

 microscope, whilst the second set is admitted, if the angular 

 aperture of the object-glass is sufficiently large ; and tlius is 

 the contrast of light produced which renders the markings 

 visible. If the aperture of the object-glass is deficient, no 

 contrast is produced, and the markings remain invisible ; but 

 the explanation of this point is the author's difficulty, and it 

 is not easy to single out in precise language his meaning. At 

 all events, the " rem invisam verba sequuntur " we cannot apply 

 to this part of his explanation, which, in so acute an observer 

 as Dr. Griffith generally is, can only be accounted for by his 

 labouring under the difficulty ot having to reconcile facts to a 

 preconceived speculative theory of iiis own. 



It is self-evident, if the tilting out of one set of rays were the 

 cause of the markings becoming visible, that this must equally, 

 and even more readily, take place under an object-glass of 

 small aperture, because not only the rays tilted out from the 

 object-glass with large aperture, but even those admitted by 

 it, as far as they exceed the angular aperture of an object- 

 glass with deficient aperture, are naturally excluded, or tilted 

 out with regard to the latter ; in fact, no rays could by any 

 possibility become excluded from an object-glass, with large 

 aperture, which were not eo ipso also tilted out from an object- 

 glass with deficient aperture : it is therefore clear, as expe- 

 rience tells us, that certain markings cannot be seen with such 

 a glass under any circumstances, that the contrast of light is 

 not produced in the manner stated, nor can the tilting out of 

 certain rays, if it takes j)/ace at all, be the cause of rendering 

 the markings visible. This objection loses, also, nothing of its 

 force when the author states, that the angular aperture must 



