130 Boss's New One-Twelfth. 



rate proportion which its angle of aperture bears to its focal 

 length, it gives good indications when a little out of adjustment. 

 These are qualities of great value. 



For exhibiting known and well-prepared objects, or for re- 

 searches in which the finest attainable definition of what is seen 

 is preferable to seeing a little more but not quite so well, the 

 new y 2 th of Mr. Ross will realize all that the most exacting 

 microscopist will desire, and no one will make trial of this glass 

 without wishing to have it. But there remains a large class of 

 inquiries for which we think Mr. Ross could provide better 

 than by his ^th. If he constructed a glass of somewhat shorter 

 focus, and of smaller angular aperture, it would, in all proba- 

 bility, be a less perfect specimen of optical skill than the ^th, 

 and decidedly inferior to it for the purposes we have mentioned ; 

 but we think it would excel it in certain common cases. It 

 would, we will suppose, be infinitesimally inferior in definition ; 

 but it would have a noticeable increase of penetration, and be 

 less affected if not in the best possible adjustment for the object 

 in view. Mr. Ross has probably done as much as is possible in 

 the way of reconciling the opposite demands for large angular 

 aperture and considerable penetration ; but it is, in our opinion, 

 often advisable to have recourse to a glass in which the pene- 

 tration is increased, and the angle of aperture diminished. 

 When you know exactly what to look for in amobject, it is very 

 easy to correct such a glass as the new ^th for the thickness of 

 covering, or the density of a layer of fluid through which 

 the object is seen ; but in viewing the interior of minute living 

 objects, or minute portions of organs that will not bear much 

 compression, it is by no means easy to regulate a very delicate 

 glass to the exact state of adjustment which, is best adapted to 

 the occasion ; and, in such cases, a small quantity of other good 

 qualities may be advantageously sacrificed . to secure a little 

 more penetration. We do not see how any one glass of deep 

 power can satisfy the two sets of demands upon them. While, 

 therefore, we accord the highest place to Mr. Ross's new j^th, 

 and freely admit the wide range of purposes for which, perhaps, 

 nothing else can be so good, we think there is ample room for 

 the employment of an obective differing from it in the manner 

 we have explained. 



