84 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
extremity of the rod. In the lower end of the screw a hardened steel 
pin is recessed so that eccentricity of the screw cannot possibly be con- 
veyed to the arm. Attached to the arm is a plate which receives the 
sleeve-tube in which the body-tube carrying the eye-piece and objective 
slides. The body-tube is of standard size, and of fixed length of 160 mm. 
The stage is fastened to the pillar rigidly, is of liberal proportions, and 
is provided with a revolving diaphragm. The dimensions of the instru- 
ment are as follows : — total length with objective and eye-pieces attached 
ready for use, 11 in.; stage, 3J x 3§ in. The outside dimensions of 
the case are 11 in. high, 4J in. wide, fij. in. deep. The inventor speaks 
highly of the portability and working qualities of the instrument, as well 
as of its suitability for school use. 
(2) Eye-pieces and Objectives. 
Formulae for Small-Apertured Objectives.* — Dr. H. Barting sum- 
marises the formulae which have been developed by him elsewhere. 
They refer to the radii of a two-membered cemented objective of given 
qualities of glass, and are constructed for water- and dry-immersions. 
o'! = the reciprocal of the working distance. 
a' 3 = the reciprocal of the focal distance from the last surface. 
r? 0 = refraction coefficient of immersion (in dry systems = 1). 
rt v n 2 = the refraction coefficients of the first and second lenses, cal- 
culated with reference to the D line. 
dn 0 , dn lf dn 2 = corresponding dispersions between lines C and F. 
Then 
II 
£ 
_ d n ° . 
n 0 * 
d n 0 
d n x 
N 0 = — - 2 
n 2 
n i * 
d n 2 
N 3 = 
n 2 
s 
a i~ + e N,L, — NjLj’ 
2 ~ “ ‘N.Li-N.L, - 
O’! — O' 3 . 
n 3 l 2 -n 2 l 3 
~ N 2 L, - Nj L 2 - 
N.L.-N,!, , 
62 — N 2 L, — M, L 2 ‘ 
* Zeitschr. f. Instrumentenk , 1898, pp. 331-5. 
