IMMERSION OBJECT-GLASSES FOR THE MICROSCOPE. 113 



If we suppose a ray of light revei'sed, and to travel down 

 the instrument, and after many crooked refractions at last to 

 be traced within the substance of the front lens having a 

 plane front, the limiting angle for the dry lens into air is 

 about 41° or 42°. On entering the stratum of air some 

 extreme rays sj^read out nearly horizontally inwards, and 

 converge to the focal point. But the moment water replaces 

 the air all is changed ; none of the rays can possibly be in- 

 clined at a greater angle than about 53° to the vertical, the 

 deviation from glass into water being, in this case, about 

 eleven degrees only for the extreme ray. 



It is Avell known that the great difficulty in making a 

 superior objective consists in managing the refractions of the 

 extreme rays striking the front lens of the objective at so 

 Avide an angle. In the immersion system no ray ever strikes 

 it at a greater angle of incidence than 53°, whilst in the dry 

 lens it is the highest ambition of the optician to make the 

 extreme ray strike it at nearly 90°, thus giving nearly an 

 angular aperture of 180°.^ 



But it is evident when the angle is limited to 106° or 107° 

 in water the minute spray of rays forming a conical pencil 

 emanating from an illuminated particle find their way vid 

 water into the objective with less violent refractions, and are 

 crowded more thickly into the central areas of the front or 

 facet lenses. 



Increased light and brilliancy are effected, as is generally 

 acknowledged. In fact, more rays from a bright particle 

 under observation in the focus of the instrument find their 

 way via ivater thaii can possibly be effected vid air, and pass 

 more centrally up the instrument. 



Another statement has been made in reference to im- 

 mersion lenses, which is entirely erroneous, viz. that the 

 thickness of the covering glass is of no importance when a 

 film of water is above it, as the diminished film of water com- 

 pensates for a thicker cover. This could only be true if a 

 ray of light passed from glass into water without deviation 

 and without aberration, i. e. if the water and glass cover had 

 each the same mean refractive index and the same dispersive 

 powers, which is far from being true. 



I have found that in the finest objectives now obtainable 

 the most exquisite definition, even of an immersion lens, 

 depends very sensibly upon having the covering glass of pre- 

 cisely the thickness and nature as that for which the glass 



' The angle of incidence is defined in optics to be the inclination of the 

 ray to the perpendicular to the surface at the point of incidence ; Iiere the 

 axis of the instrument. 



