Immersion Lenses and New Befradometers. 73 



reflexion of light may also be illustrated by means of the following 

 apparatus : — 



" A glass chimney is fastened into the bottom of a rectangular 

 vessel of glass, ah cd, which is filled with water, and a candle 

 introduced into the chimney. The rays from the candle are seen 

 to emerge from the water when the angle of incidence is less than 



3 



the angle whose sine is -^ ; but when the incidence exceeds this 



limit, the light is seen to be totally reflected, and none passes 



3 

 through the surface of the water." The angle whose sine is j 



of course equals 48° 35' nearly. Now will the reader suppose that 

 a layer of oil of cassia is poured upon the surface of the water, then 



for cassia, ju. the refractive index is 1 • 635 

 for water, a „ „ 1"336 



for the line E (of the spectrum of Fraunhofer) of mean refran- 

 gibility. Evidently it is quite impossible that the angle of total 

 internal reflexion can be the same as before, when the ray, emerging 

 from the water, strikes the under-surface of the cassia. 



The value of - is now 1 -336 -^ 1 -635 = -8171. 



The angle whose sine is - = 54° 48', which is a great increase 



on 48° 35'. For water and glass it is, as we have seen, about 62°.. 



The most important part of the subject is unquestionably — 



III. Definition. — The writer cannot but regret that he has had 

 the misfortune to incur the displeasure of Mr. Wenham in making 

 the following statements in the interest of Microscopical Science : — 



" Even in a good -^th object-glass the spherical aberration does 

 not exceed the 50,000th of an inch." 



" The most difficult definition in the Podura is the substratum 

 of beads glimmering through the membrane nearest the hght. . . . 

 The beading dispels the mist and haze always accompanying the 

 spurious ' spines.' " 



" That our old-fashioned glasses are wrong somewhere — in the 

 correction of spherical aberration." 



" That the battle of the glasses will have to be fought ; and 

 that the superiority of the immersion lens is an u-refragable proof 

 of the corrections necessary." (Dec, 1869.) 



" That in viewing the image of a flame in a minute globule the 

 spurious disk is more than double the size of the true size of a 

 perfect aplanatic image." 



" That in the diameter of this spurious disk lies the whole pith 

 of the objective corrections." (Dec, 1870.) 



