MEMORANDA. 239 



accompanying duplicates, one of which being mounted dry, 

 that is, with the cover merely supported by its edge, will 

 show the delicacy and little risk there is in cracking the thin 

 glass. In examining a slide, it will, of course, be necessary 

 to focus for the upper surface of the cover first, until the 

 circle be found, when, on lowering the object-glass, the 

 specimen will be seen in the field, if the light in both cases 

 has been central. — W. K. Bridgman, Noricich. 



On the Aperture of Object-giasf^es. — Having read over the re- 

 marks in your last number on the Aperture of Object-glasses, 

 by my friend Mr. Wenham and Dr. Robinson, I should wish 

 to offer a few remarks ; not that I shall attempt to take up the 

 valuable pages of your Journal by discussing the matter in 

 the two papers, but I should wish to call attention to a par- 

 ticular fact connected with a well-conducted experiment 

 named in my last communication, and which neither Mr. 

 Wenham nor Dr. Robinson have noticed. For if it be a fact 

 in one case that the angle of aperture of an object-glass be re- 

 duced when brought to bear on an object mounted in balsam, 

 it must be so in every case. The experiment which I refer to 

 is one which I have again tried with great caution, and with 

 the same result. 



Let a a be a pencil of light falling upon the under-surface 

 of the anterior lens of a set of wide aperture, say 152°, and 

 let the central ray of the pencil a a make an angle of about 

 75° with the axis of the lens h b ; take two sliders, the one con- 

 taining an object mounted dry, and tlie other an object mounted 

 in balsam, and let them be so selected that the object in both 

 sliders may be exactly in focus when placed under the ob- 

 jective, without having occasion to move the adjusting screw. 

 Now, when the pencil of light makes an angle so great as 

 above stated, a part of the field of view will not be perfectly 

 illuminated ; place the slide with the dry object under the 

 objective, and it will be found that the field is still partly 

 illuminated as before ; then remove this slide and place the 

 one below containing the object mounted in balsam, the field 

 is still invariably illuminated in the same manner, there being 

 no diiference in the illumination, however the rays may have 



