86 MEMORANDA. 



experiment, that I ought not to have seen the markings on 

 delicate test objects, when mounted in balsam." From this 

 I infer that Professor Bailey had not seen a paragraph con- 

 tained in my communication, in the ' Quarterly Journal of 

 Microscopical Science' for January, 1855, page 162, or I feel 

 assured that he would not have thought it necessary to make 

 tliis form of reply, for I therein assert that subsequent expe- 

 rience had induced me to recall my remarks, and that I had 

 lately succeeded in bringing out the striae of some very 

 difficult tests when in balsam. I will now corroborate this 

 by saying that I am convinced that Professor Bailey is per- 

 fectly correct in his statement with respect to balsam tests, 

 which must henceforth be recorded in the list of facts. Thus 

 far we are quite agreed ; but as Professor Bailey's allusions 

 extend beyond this point, self-defence will be my apology for 

 taking some notice of them. Referring to me. Professor Bailey 

 says, " 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 i\ie balsam instead of into it, as in Mr. Wenham's 

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

 aperture are as useful for balsam-mounted specimens as for 

 others." Surely Professor Bailey cannot have well considered 

 this extraordinary, because extremely incorrect assertion, 

 which is tantamount to saying, that a diverging pencil of rays 

 from a luminous point, submerged in balsam, will in each 

 case continue their course in the same right line, without 

 suffering any refraction, after emerging from a plane surface 

 of the medium. This is contrary to all reason, for in the 

 trigonometry of optics where there are sufficient data con- 

 nected with the position and direction of the rays, it comes 

 to precisely the same thing whether they are traced into the 

 refractive medium or out of it. But taking Professor Bailey 

 on his own statement, I will explain what is the real effect in 

 this case. Suppose a series of rays diverging from a balsam- 

 mounted object ; from the mean refraction of the balsam and 

 glass cover (the indices being about 1*54 and 1-53) total 

 reflection would take place from the upper surface of the 

 latter at an angle of very nearly 41° from the perpendicular. 

 This, therefore, at once limits the angle of rays collected by 

 the object-glass to 82°, and as total reflection begins where 

 refraction ceases, all rays beyond this point will be entirely 

 reflected down again into the balsam, and lost by dispersion ; 

 and the extreme rays of the pencil of 82° that just exceed 

 total reflection by passing through the glass, so far from con- 

 tinuing their course in a straight line, are brought down by 

 refraction to the very level of the top surface of the cover 



