142 SUMMARY OF CURRENT RESEARCHES RELATING TO 
The above method gives a greater range of readings for indices 
varying from 2 to 2-5, and consequently more accurate results than the 
simpler one of filling up a plano-concave lens with the medium and 
covering it with a piece of plane glass. The formula for this latter 
plan being 
' i r 
* ‘ 
The radius of the concave r might with advantage be made 2 in., 
then 
4- 2 
If /* = g’ anf l F = co , /x' = | ; if F = 4, /a = 2 ; and if F = 2, 
= 2-5. 
(6) Miscellaneous. 
Experiments on the Diffracting Structure of Striated Muscle- 
fibre.* — Dr. O. Zoth has made a series of experiments on the diffraction 
of striated muscle-fibre similar to those first undertaken by Ranvier. 
He supplements the simple method of observation employed by 
Ranvier with the method of Abbe, by which the diffraction spectra are 
observed in the Microscope, and uses the following arrangement. The 
source of light was at first a vertical slit, 10 mm. long and 1 mm. 
broad, in the screen of an argand burner, but later this was replaced 
by a zircon thread ignited by a Bunsen flame. Behind the Bunsen 
burner is a black background, and in front of it a black diaphragm 
with rectangular aperture, which serves to permit of the measurement 
of the bright line and to cut off extraneous light. The light from the 
source was reflected from a plane polished steel mirror, and a real 
image of the bright line was formed above the Abbe condenser. With 
the ordinary distance of 30-40 cm. between line and mirror the con- 
denser (Zeiss 1‘2 Ap.) throws an image of the line above the object. 
The Microscope, with low magnification (Zeiss A, eye-piece 2), is first 
adjusted on the object ; the body-tube is then gradually raised until 
the image of the line is clearly defined, when on both sides of this 
direct image the diffraction spectra become simultaneously visible. 
For the measurement of the distances of the individual spectra it 
was considered sufficient for the author’s purpose to determine by 
means of an eye-piece micrometer the linear distances between the 
centre of the undiffracted image and the centre of the yellow in each 
spectrum. 
The sartorius muscle of the frog was made the chief subject of 
experiment, and the numbers obtained by the above method in this 
case were compared with those resulting from similar observations on 
insect muscle. The arrangement employed was : Zeiss objective A, 
eye-piece III., with micrometer 5 mm. in 50 divisions, body-tube 155 
cm., condenser 1‘2 Ap., distance of source of light from the centre of 
the mirror 35 cm. The numbers obtained in divisions of the micro- 
* S.B. Akad. Wiss. Wien, xcix. (1890) pp. 421-43. 
