46 DR. F. D'ALQUEN ON ; 



already noticed before. Moreover, it must not have appeared 

 quite conclusive to the author himself, because he continues : 

 " Or to simplify this most important point, the object may be 

 regarded as illuminated by two sets of rays, one correspond- 

 ing to those admitted by the object-glass of the smaller aper- 

 ture, the other set, to these plus those admitted by the excess 

 of angular aperture of t!ie second over the first." Now we 

 may not only regard with the author the object as thus illu- 

 minated, but we know that such is actually the case, and that 

 the efficacy of the larger aperture over the lesser depends 

 simply on the admission of additional rays which were too 

 oblique to enter the latter ; but simple as this is, we must ask 

 again, how can the admission of additional rays, here assumed, 

 prove the tilting out of others ? Mark the answer : the first 

 set not being sufficiently oblique to allow a portion of them 

 being refracted beyond the angular aperture of the first object- 

 glass, while the second set are so. Now every one will admit 

 that this illustration proves nothing, because the rays admitted 

 by the first object-glass are as oblique with regard to its 

 angular aperture, and the practicability of becoming tilted out 

 as the rays entering the second to its corresponding larger 

 aperture. 



Another objection which is, a priori, as palpable as those I 

 have already noticed is this : if the markings are rendered 

 visible, by the tilting out of certain rays, it would follow, 

 as the fewer rays will be tilted out the greater the aperture, 

 that the markings, instead of becoming more distinct, must have 

 their distinctness impaired in the same proportion as the 

 angular aperture is increased ; yet experience tells us that the 

 reverse is the case. If we further assume that the illumina- 

 tion remains the same, the more we increase the angular aper- 

 ture, the more it would become impossible to realize the 

 alleged conditions for rendering the markings visible ; and 

 with every degree added to the aperture, the markings ought 

 to get fainter, which is contrary to the fact. Lastly, from the 

 excessive minuteness of the depressions, it appears to me 

 highly improbable that the difference thereby occasioned in 

 the thickness or substance of the valve should be the cause of 

 giving a different refractive index to different portions of the 

 valve ; and I feel more inclined, with other observers, to attri- 

 bute the modification which the light undergoes, on passing 

 through it, to pecidiarities in the structure of the markings 

 themselves. However, let us proceed from assertions and 

 counter assertions to practical experiment, the ultima ratio 

 in an inductive science. 



By means of a small pipe of an injecting syringe, with an 



