ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
651 
(S) Eye-pieces and Objectives. 
Watson and Sons’ “Holoscopic” Eye-pieces. — 
(fig. 155) are on the Huyghenian principle ; the 
eye-lens is attached to a draw-tube, to the lower 
end of which the diaphragm is fixed. When the 
tube is pushed in as far as it will go, the eye- 
piece is under-corrected ; but, as it is extended, it 
becomes increasingly over-corrected. The latter 
condition enables it to be used with apochromatic 
objectives, and the former with the ordinary achro- 
matic lenses. They have been specially designed 
to obviate the necessity for two sets of eye-pieces 
if a worker has both apochromats and achromats in 
his outfit. 
Spitta, E. J. — “ Achromatics verms Apochromatics.” 
Amer. Mon. Micr. Journ., Oct. 1899, pp. 296-309. 
The author gives a clear popular article on the subject. 
These eye-pieces 
Fig. 155. 
(3) Illuminating 1 and other Apparatus. 
Notes on Optical Projection.* — After some introductory remarks 
upon the unfamiliarity with the principles of lantern projection fre- 
quently displayed by lecturers, Prof. Behrens describes two contrivances 
he has invented for the improvement of his projection apparatus.f 
I. Electric Hand Regulator for Microscopic Projection. — He has made 
inquiries at various Institutes using his apparatus, and has found, with 
regard to the relative merits of lime-light and electric light, that the 
former, if used with his burner, is perfectly sufficient even for very large 
audiences when glass pictures are projected ; and that it is even pre- 
ferred to the violet arc light on account of its warm pleasant tone. But 
in the case of the projection of microscopic slides, a light stronger than 
the ordinary lime-light was desirable. On testing the following light 
sources with Weber’s photometer, it was found that their brightness 
measured in meter candles was, — Schmidt and Haensch’s zirconium 
burner, 12 ; Behrens’ lime-light burner, 26 ; Schuckert’s differential 
lamp with 20-ampere current, 125. 
The superiority of Schuckert when high illumination is required is 
manifest, and the author has therefore adapted his apparatus for its 
reception, and in so doing has made use of the lamp alone without its 
heavy brass and iron fittings. He has moreover mounted the lamp in a 
light aluminium camera. In his hand regulator he has endeavoured to 
remedy the usual defects of arc lights, viz. their uncertainty, costliness, 
want of adaptability to other than their assigned current intensity, and 
want of interchangeability with a lime-light burner. 
He has every reason to believe that his hand regulator thoroughly 
satisfies the following eight conditions : — (1) It is exchangeable for a 
lime-light burner ; (2) it is applicable both for a constant and a variable 
current ; (3) it is applicable for currents of the most varying intensities ; 
(4) the current flows only through the carbon and not through the 
* Zeitscbr. f. wiss. Mikr., xvi. (1899) pp. 183-95 (3 figs.;, 
t Cf. this Journal, ante, p. 89. 
