Immersion Lenses and New Refradometers. 69 



have, accordingly, for many years been using them ; whilst the 

 English have been struggling with the formidable difficulties of 

 correcting the glasses for aerial refractions ; the film of air making 

 enormous differences in the course of the excentrical pencils.* 



For whether we trace the rays downwards through a microscope 

 to the object, or upwards from the object, no possible difference is 

 made in the course of the ray so long as the conditions of the 

 media remain the same. 



A ray radiating from an illuminated particle immersed in the 

 balsam and striking against the surface of the air above the cover, 

 is totally reflected at an angle of about 42° ; but substituting 

 water for air at about 62^, as follows from Ptolemy's discoveries. 



To use the words of one of the most distinguished writers on 

 Optics in this country, to whom the writer submitted the prin- 

 ciple of the water lens, advocated in these papers, a much greater 

 number of rays from the object reaches the eye of the observer 

 via water, than can possibly take place via air. 



But not only do more than double the quantity of rays reach 

 the eye through the objective and eye-piece, but they find their 

 way more directly without such violent bending or abrupt refraction. 

 This advantage is inestimable. For, as is well known, the aberra- 

 tion of a prism is a minimum only when the deviation is the least 

 possible, and all lenses act on the principle of the prism. 



The celebrated Huyghens invented his eye-piece entirely upon 

 the principle of dividing the refractions as nearly as possible, so that 

 each lens should bend the rays equally at a minimum deviation. 

 Fortunately, and unexpectedly, the combination of the focal lengths 

 of the lenses as 3 : 1, and separating the lenses by a distance equal 

 to half their sum, also produced an excellent achromatism for 

 parallel rays as it were by accident. 



Messrs. Powell and Lealand have informed me that in using the 

 greatest obliquity of illumination attainable by their peculiarly- 



* As it seems right in the eyes of some of the old lovers of dry objectives to 

 dispute a number of things which they did not learn in their youth, it may not 

 be out of place here to quote a principle, at which, doubtless, advanced optical 

 students are quite au fait. The aberration of the direct ray refracted through a 

 plate " of thickness (0, is equal to the thickness multiplied by imity divided by 

 the refractive index. 



t 1 



i. e. = — and — = sine of angle of total reflexion. (See p. 72.) 



2 3 



For plate glass, it is therefore - the thickness : for a film of water it is — the 



thickness. But the dispersion is very different in the two cases: for oblique 

 rays the aberrations for obliquity 6 are respectively for thickness {t = 1) 



2 3 



— secant and — sec. d, 



3 4 



and enormous differences in the paths of the excentrical rays necessarily result." 

 (See Parkinson's ' Optics,' p. 79.) 



