118 The Refractive Powers of Peculiar Objectives. 



object in balsam. The ray r, identical wifcli that of Fig. 2, having 

 passed through the liquid above the cover, and the cover itig glass 

 itself, will, or a ray correspondent to it, proceed to the convex 

 surface of the cemented plano-convex lens below the slide, and 

 emerge there as traced in the figure, or substantially so. An 

 arrangement for procurement of 130 is indicated in Fig. 1, where 

 the dotted lines show a sharper curvature as also thinner lens. 

 When this lens is moved towards the system next back of it until 

 the surface meets the ray r, coincidently with the dotted hne r', 

 then that ray can have refraction correspondently with the line 

 drawn by Mr. Wenham for 130^ in his dry Jth diagram. This 

 can be strictly so when the medium between ths plane surface of 

 the front and covering glass is of the same refractive index as the 

 glass. In this case, too, no limitation by total reflexion. But if 

 water is used, there would be limitation, and within 130^. To 

 gain fully that aperture, a medium of somewhat higher refractive 

 index than water would be needed. But only the refractive ability 

 of the connecting medium is the real practical limit of angle of 

 transmitted pencil in the immersion objective below maximum 

 angular aperture. 



The limitation takes place at the upper surface of the covering 

 glass, not necessarily anywhere else, with balsam-mounted objects. 

 If air is the medium in the interspace above cover, only 82" 

 approximately can be obtained, with whatever appliances. 



As stated in my first communication, an immersion objective, 

 if used below the slide as condenser, or any form of immersion 

 condenser of sufficient angle, has the same action (of course) as 

 the plano-convex lens, to increase the angle of the interior pencil, 

 according to the immersing medium used. 



Mr. Wenham, in his last note,* asks some questions, as follow : — 

 " Does he" (the writer of this) " really expect us to believe that 

 he can obtain all his aperture by the back combinations alone?" 

 and in response I have described a means of procuring 130^ with 

 three " back combinations " in use instead of two. 



Again, " or that any of his objectives, when duly adjusted 

 for an immersed object, and thus showing a large aperture when 

 measured in air, will retain the same angle when the front is 

 immersed in water ? " ! Certainly not (vide my own experiment 

 described in the first dozen lines I ever wrote on the subject, 

 ' Mon. Mic. Jour.,' No. xxxi., p. 37, where an objective of 170°, 

 measured in air, is credited with only 100° when immersed in 

 water). By the way, Mr. Wenham in his own experiment f reaches 

 the same result, viz. "a ^Vth of 170'^" in air, in water showed an 

 " apparent aperture" of 100°. The objective being built for water- 

 immersion use (doubtless) that was the proper medium for the 



* 'Mob. Mio. .Jour.,' No. xxxvi., p. 292. f Ibid., No. xxxii., p. SH. 



