1877.] Microscopy. 631 
try round Lake N’yassa. In the same number is an extended notice, by 
Keith Johnston, of the cartographical publications of the Indian Survey. 
These include, besides numerous maps of special districts, the “ long- 
expected ” General Map of India. A book relating to the discoveries of 
the fifteenth century has been published by Richard Henry Major, en- 
titled The Discoveries of Prince Henry the Navigator, and their Re- 
sults; being the Narrative of the Discovery by Sea, within One Cent- 
ury, of More than Half the World. This period includes the explora- 
tion of the coasts of Africa, the discovery of America and Australia, the 
circumnavigation of the globe, and the opening of a sea-way to India, 
the Moluccas, and China. J. W. Boddam Whetham, in a book entitled 
Across Central America, gives interesting notes of travel through a hith- 
erto rarely visited region, with an account of some of the wonderful ruins 
of Central America. 
MICROSCOPY.! 
E. Guxpiacu’s New Periscoric Eye-Prece.— The Hughenian 
eye-piece, as originally constructed, consists, as is well known, of two 
plano-convex lenses, of which one, the field-lens, has three times the fo- 
cal length of the other, the eye-lens, the distance between the two be- 
ing equal to double the focal length of the eye-lens, the plane side of the 
field-lens facing the convex side of the eye-lens. 
The field-lens not only widens the field of view but also corrects the 
Spherical as well as the chromatic aberration, as it is placed beyond the 
focal distance of the eye-lens (which is the actual eye-piece), and in con- 
Sequence thereof acts negatively to the same. 
This correction, however, is not a complete one, for with the most fa- 
vorable distance between the two lenses a not inconsiderable remnant of 
the chromatic aberration still remains, while the spherical aberration is 
already correspondingly over-corrected. The first is noticeable by the 
blue edge bordering that side of the object which is turned toward the 
centre, when the object is placed towards the edge of the field; the 
remnant of the spherical aberration causes the distortion and want of 
sharpness of definition at the edge of the field. By increasing the dis- 
_ tance between the field-lens and eye-lens the blue color may indeed be. 
made to disappear, but the spherical aberration increases correspond- 
ingly, and the field is narrowed considerably. If, on the contrary, the 
field-lens is brought closer to the eye-lens, the spherical aberration is 
certainly diminished ; but notwithstanding this, the image at the edge of 
the field does not become any more sharply defined, because the chro- 
Matic aberration has increased in equal ratio. 
One advantage, however, is gained by approaching the field-lens closer 
to the eye-lens, namely, a considerable widening of the field. 
If, under these circumstances, the aberrations of the eye-lens are cor- 
1 Conducted by Dr. R. H. Warp, Troy, N. Y. 
