116 The Ihfractive Foivers of Peculiar Ohjedives. 



transmitted will be increased, and for this case Mr. Wenbam's 

 challenge cannot stand. 



For the sake of the simplest, clearest exposition I have in design 

 as related made my immersion objective for illustration of four 

 systems, borrowing Mr. Wenbam's dry ^th diagram in part as before 

 stated. It only now remains to point out the variation necessary 

 to construct the immersion objective of three systems and of ap- 

 proximate, interior angles. 



For this purpose I will adhere to the same |th in. diagram, and 

 indicate the modification in front lens needed to accomplish the 

 result. 



In the Fig. 2, the outside dotted line indicates the convex sur- 

 face of the dry front as drawn by Mr. Wenham ; the continuous 

 line of a shorter radius within that is to indicate the curvature 

 necessary to produce a |th-inch immersion of angle about 105'^ 

 interior pencil. What is intended is to increase the refraction of 

 the convex surface of the front by sharper convexity, or higher re- 

 fractive material, or hoth, to the extent necessary to make up for 

 the diminution at the plane surface which takes place according to 

 the refractive power of the medium flowed in. " The mere fusing 

 by a refractive medium of the thinner or immersion lens and cover 

 into one," as Mr. Wenham says; but here with the difference that 

 the front lens remains of the same thickness as the dry, while the 

 convex surface has (according to requirement of the case) increased 

 refraction. 



Below the new front of this now immersion -J;th is represented 

 the thin cover and thin slide (increased about four times with, 

 understood to be, balsam-mounted objects between. The ray r, 

 Fig. 2, may be continued — tracing the ray from above in this case 

 — to the object under the cover, because, although in the case of 

 water there is some positive refraction at the plane surface of the 

 front, it is exactly, or very nearly, balanced by negative refraction 

 of the first surface of the covering glass, viz. tracing the ray from 

 above, downward towards the object, as Mr. Wenham does in his 

 treatment of the subject in his diagram. ^Moreover, if the flowed - 

 in medium were of the same refractive index as the glass, the line 

 would be straight. Well, in continuing this ray of 52-|^ incidence 

 to the lower surface of the slide, the law of total reflexion (interior) 

 comes into effect. Of course only 41^ incidence, and 82^ angular 

 pencil, will have passage through the lower surface of the slide. 

 This total reflexion boundary forms an absolute limit of angle for 

 objects in balsam. I might refer my critic to my first communica- 

 tion* for the proper method to adopt to gain a larger pencil, but 

 otherwise and easier let reference be made to Fig. 3 annexed, in- 

 tended to represent an ordinary object-slide with covering glass, and 



* ' !Mon. Alic. Jriir.,' Xn. x\xi.. p. SG. 



