302 MEMORANDA. 



Grammatophora, but in the evening we saw both these 

 objects [balsam-mounted] satisfactorily resolved into dots by 

 unreflected oblique light from one wick of a common bed- 

 chamber lamp, burning oil, a homely but very eflFective 

 method of illumination for objectives of large apertures." 



It appears then that the resolution of balsam-mounted spe- 

 cimens of difficult test-objects can be accomplished, in spite 

 of Mr. Wenham's arguments and experience to the contrary. 

 The error in his arguments will be sufficiently obvious to any 

 one who will trace the course of a divergent pencil of rays 

 out of the balsam instead of ijito it, as in Mr. Wenham's 

 experiments, and it will thai be seen that large angles of 

 aperture are as useful for balsam-mounted specimens as for 

 others. I leave the defence of large angles of aperture to the 

 professed optician, being well satisfied that, notwithstanding 

 the extraordinary attempts made by certain writers in Eng- 

 land to underrate the value of the improvements made in this 

 direction, no one who has once employed a properly-corrected 

 ■object-glass of large aperture will ever be satisfied with one 

 of a different construction. — Professor J. Bailey, in Ame- 

 rican Journal of Science and Arts. 



Apcrtiii-e of Object-Ci lasses iu relation to Objects in Canatla 



■Balsam. — In Continuation of this subject I have to bring for- 

 ward a few experiments recently made, for the purpose of 

 viewing objects with the full aperture of the object-glass, 

 when mounted in Canada l>alsam.. 



The fact that bakam does diminish the angle of aperture 

 of the microscopic object-glass when in action for viewing a 

 structure, mounted in the substance of that or any other 

 refractive medium, has already been sufficiently demonstrated, 

 both theoretically and practically, in the papers of Professor 

 Robinson and myself; and I consider that no further proofs 

 are requisite for establishing the truth of a position, which a 

 few simple experiments can be made to convey the most 

 direct conviction to unassisted eyesight, and which is so 

 strictly based uptMi the very first laws of incidence and 

 refraction. A fact so obvious as this can only be disputed 

 for two reasons — either ignorance of these laws, or a doubt of 

 their accuracy. To argue the point on one or both these 

 grounds would be beyond my province or capacity ; I, there- 

 fore, take this as a reality not to be disputed, and as such 

 proceed on the strength of it. 



The sharpness and beauty with which some test-objects are 

 displayed under the diminished aperture, consequent upon 

 baJsam-mounting, is upon first consideration rather surprising. 



