70 Immersion Lenses and New Refraeiotneters. 



mounted condenser, it was only when the immersion lens was used 

 that the extremely oblique light from the object conld enter the 

 object-glass and pass to the eye ; the object being mounted of course 

 in Canada balsam. If, indeed, we plunge the objective into Canada 

 balsam, to view an object immersed in it (as proposed by Mr. Wen- 

 ham, page 18, line 7), in order to demonstrate the principle of the 

 water lens by applying it without the water to prove its action 

 with it (save the mark !), though they have very different refractive 

 effects — why we can thus certainly imagine both effects are alike.* 



It cannot be too clearly asserted that no dry objective, whatever 

 be its aerial aperture, can receive within the substance of its front 

 lens rays more oblique that 42°, nor with an immersion film of 

 water more oblique than 62°. And, even then, the ray in both 

 cases must strike the face-glass at an obliquity of nearly 90°. Yet 

 Mr. Wenham says they are both alike ! It would be unkind to 

 suggest that Ptolemy's optics, nearly 2000 years old, have an 

 advantage, as regards immersion lenses, over Mr. Wenham's. I 

 commend the Ptolemy immersion lens to his especial cogitation. 



11. Eefractions. — In so interesting a question, the teachings of 

 Nature herself should not be neglected. " The crystalline lens " is 

 an " immersion " placed in two refracting fluids, possessing an aper- 

 ture of above 100°. In fishes, the view under water is worth con- 

 sideration. To the gaze of the fish the whole prospect of earth and 

 sky is contracted within a cone of which the apex is the eye and 

 the base a circle of about 96° angular aperture, beyond which 

 many objects, as its fellow fish, appear double by total internal 

 reflexion. t The angler on the bank of the river is seen in minia- 

 ture. As objects approach the zenith, they enlarge rapidly. All 

 the objects on the horizontal plane are crowded into a minute 

 annular space. 



If we hold a tumbler of water steadily above the level of the eye, 

 any substance floating in the water may be seen internally reflected 

 at the upper surface more brilliantly than any metallic surface can 

 effect it, by reason of the total internal reflexion (Parkinson). This 

 phenomenon beautifull}^ explains the great loss of pencils radiant 



* It is harcily necessary to inform the reader that the Plate LX. is drawn 

 merely to illustrate the comparative angles of total internal reflexion in the case 

 water verstts air lenses, as also the deviations and aberrations of the rays upon 

 entering the first glass front of the objective. Indeed, the curves of the upper 

 lenses are quite unnecessary for my purpose ; and the extremely minute scale of 

 the refractions at the focus, when sulflciently enlarged, left no room in the plate 

 to draw the lenses correctly. Mr. Wenham very properly points this out in his 

 stricture on page 17, January, 1871. But this does not in the least affect the 

 question of comparative refractions between the brilliant particle and the first 

 surface of the objective : the real point at issue, in the case of dry and immersive 

 fronts. 



t The aquariums (now destroyed) exhibited very brightly the images of the 

 gold and silver fish totally reflected by the upper surface of the water to the 

 visitors at the Crystal Palace, who looked upwards through the side. 



