196 American Quarterly Microscopical Journal. 



Mr. Wenham, in a recent article on the " Measurement of 

 Angle of Aperture of Objectives," in the Journal of the Royal 

 Microscopical Society, December, 1878, says, in reference to the 

 large angles of aperture claimed for immersion objectives: 

 *'We are not seeking for foci within the front lens, or yet on 

 its surface," and he further remarks, "that every lens he had 

 seen, professing 180°, does, in fact, focus on dry objects." 



If I understand him, he admits that objectives on the 

 immersion principle may, by immersing the object in what 

 is really, for the time being, a portion of the lens itself, have 

 an angle in balsam of 82° + , or more than twice the critical 

 angle. 



If Mr. Wenham, however, intends to say that because every 

 objective he had seen, which was marked 180°, did in fact focus 

 in air, these objectives could not have as much as 82°, balsam 

 angle, to say nothing of more, then I must dissent. I may be 

 wrong in so understanding him, and do not wish to be con- 

 sidered as positively asserting that he does imply this ; but 

 such is the inference that I draw from his words. A reference 

 to Plate XVII., Fig. i, will explain how an objective, whose 

 true air angle is only 144°, may actually pass 87° in glass 

 (balsam); a is the glass front of the objective, of a certain 

 thickness ; b the interposed immersion fluid, and c the cover, 

 under which is the object e in balsam. We will suppose glass, 

 fluid, balsam, and cover to have the same refractive index, 

 and that when the objective is in focus, e is at the distance 

 shown in the diagram from the curved surface a. It is quite 

 evident that it is a matter of indifference, in the present case, 

 whether the glass part or lens should be a little thicker, 

 and the fluid or the cover thinner, or vice versa, so long as 

 the distance from the curved surface to e remains unchanged. 



Rays e, d, and e,f, diverging from e at an angle of 87^", will 

 emerge at the posterior or curved surface a, without refraction, 

 if e is in the center of curvature, or they will be refracted if e 

 does not coincide with that point. 



Supposing, now, we were to replace the fluid and cover by 

 glass, /. e., make the whole of glass as thick from « to ^ as 

 it is when used as an immersion, it. would no longer be pos- 

 sible to focus it; if e was uncovered and infinitely small, it 

 would be just on the flat surface of the lens. By removing 

 the fluid portion, however, and not replacing its thickness 



