120 Further Remarks on Immersion Apertures. 



In the next place, I cannot assent to his implied claim that he 

 was the originator of four-combination immersion objectives possessed 

 of a balsam angle greater than 82^. In the experiments on which 

 this claim is based, the hemispherical lens was united to the cover 

 by balsam, and entirely detached from the objective. If the idea 

 that the nearly hemispherical front of an objective, united to the 

 cover simply by water-contact, might play a similar role in trans- 

 mitting pencils greater than 82° from a balsam-mounted object, 

 ever occurred to him before I described the device of Mr. Tolles, in 

 my paper last June, he has certainly not put it on record. 



Even now he seems still unwilling to believe the possibility of 

 the transmission of a pencil greater than 82^, if the objective has but 

 two posterior combinations, although he sees that it is easy enough 

 if it has three. I am glad, however, to perceive that he is almost 

 persuaded, for he writes : " The capabihty of taking in a few extra 

 rays may depend upon the form and size of the back lenses," 

 which is substantially what I urged in my November paper. 



To the argument of that paper he makes, if I fully understand 

 him, but one objection, and that, I must say, appears to me to 

 involve an essential oversight. I refer to the first paragraph on 

 page 257, in which he objects to my assuming successively two 

 different focal points for the same front, saying that " if F is 

 considered right, F' must be wrong." Now, surely, no one knows 

 better than Mr. Wenham, although he appears to ignore it al- 

 together, that the distance of the focal point from the front of an 

 objective with a screw-collar is not a fixed quantity, but has a 

 maximum and minimum, with an infinite number of intermediate 

 values. Every time the screw-collar is turned the fine adjustment 

 of the microscope has to be moved. A separation of the two pos- 

 terior combinations from the front enables the objective to be used 

 with a lower magnifying power and angle; an approximation of 

 the posterior combinations to the front enables it to be used with a 

 higher magnifying power and angle than is obtained at the inter- 

 mediate positi(jns. Of course, if the object viewed is uncovered, the 

 posterior combinations can only correct the aberrations of the front 

 in one position, which is recorded on the screw-collar as the " un- 

 covered point " ; when the screw-collar is turned from this position 

 a covering glass of suitable thickness must be introduced. It is 

 clear, therefore, that to say "if F is considered right F' must be 

 wrong," without knowing the construction and position of the 

 posterior combinations in the case under consideration, is to make 

 a mere assumption for which there is no foundation, and which is 

 not strengthened in the least by the equally unfounded assumption 

 that " the first position forms a posterior focus, the second does not." 



In the paragraph following this dogmatic assertion he asks me a 

 question, which, perhaps, I do not rightly comprehend, for it is not 



