144 MICKOSCOPIC VISION. 



in his eagerness to air his Greek, has forgotten the meaning 

 of the word spherical. This figure is contrasted with the next, 

 which is said to show " Aero- Spherical Aberration." Here 

 the rays are traced from a radiant similarly placed through 

 the air film and into the flint glass of the ordinary double 

 front of a dry objective, and here again we have the aberra- 

 tion due merely to the two plane surfaces. No spherical 

 aberration is illustrated in those figures, but only those 

 aberrations arising from plane surfaces; these aberrations 

 are shown to be ever so much greater in the case of the dry 

 double front than in that of the immersion single front. 

 In practice, however, the very reverse is the case, because 

 if the proper curves were given to the dry objective front, 

 the total aberration arising from the two spherical as well 

 as the plane surfaces would be less than that of the water 

 immersion. For in the single-fronted immersion objective 

 the idea was then, and is now, to compensate the under- 

 correction of the front by the over- correction of the backs : 

 whereas in the old dry lenses the aim w^as to correct the 

 aberrations as far as possible in each separate combination 

 of the lens, hence the raison d''etre for -triple fronts. In 

 this paper Dr. Pigott proposes the use of turpentine as an 

 immersion fluid, because its refractive index is near that of 

 crown glass or balsam. This is, so far as I am aware, the 

 first suggestion of what is now known as homogeneous 

 immersion. 



We see, therefore, that in the midst of much error Dr. 

 Pigott gave to the world several important truths, viz. : — 



1. That a water-immersion lens can have a greater aper- 

 ture than any dry lens, and similarly a homogeneous than 

 a water immersion. 



2. That illuminating power is increased by the use of 

 higher refractive media ; and 



