324 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the sine condition, viz. /^/sin </> = a constant. With parallel light- 

 incidence this constant is the actual focal length of the condenser ; h is 

 the half-aperture of the iris and sin <j> is the aperture of the condenser 

 for dry systems, n sin <^ for immersion systems. The opening of the 

 condenser is, therefore, proportional to the numerical aperture of the 

 condenser and, therefore, also of the objective. On this proportionality 

 of the magnitudes depends the convenient arrangement that marks indi- 

 cating numerical apertures ; • 1, • 2 up to 1 • 3 are engraved on the rim 

 of the iris at equal intervals (fig, 45). The calculation and attainment 

 of the said range of apertures is therefore, on account of this property, 

 very much facilitated. If a preparation is placed between the condenser 

 and the objective, the emergency angle (^ of the condenser still becomes 

 the incidence angle of the objective, because object-slide, cover-glass, and 

 intervening space being plane-parallel do not affect the angles, and their 

 materials are unimportant so long as the surrounding media (air, water, 

 oil) remain unchanged. 



In determining the aperture the objective is first directed, in the 

 usual way, with plane mirror and diffused light, on to a preparation : 

 the ocular is then removed and the operator, looking directly down the 

 tube, observes the light circle in the rear plane of the objective and opens 

 the iris until the image of the iris-opening coincides with the light 

 circle. The values obtained by this method differ but very little from 

 those attainable by very precise apparatus. The convenience of always 

 having at hand a ready means of noting the conditions of aperture under 

 which a preparation yields its best results, needs no commendation. 



Simple Screw-micrometer.* — C. Barus has designed a simple screw- 

 micrometer in which the lugs are made of indurated fibre instead of 

 metal. A mirror-attachment fastened to the end of the screw is an 

 important part of the construction. The author considers that there is 

 no doubt that a screw satisfying the requirements of the interferometer, 

 and trustworthy to about 0*0001 cm. and a length of even 1 ft. or more, 

 could be constructed by the above method. It is necessary to begin 

 with a straight rod for this purpose (or preferably with a tube) of a 

 larger diameter, say 1 or 2 cm. The adjustable mirror at the end should 

 be light. The adjustment screw with orthogonal axes must be of very 

 fine pitch, if the image is to be stationary. Such a screw of low pitch, 

 carefully cut in brass and running in sockets of indurated fibre, can be 

 made in almost any laboratory. The final advantage of this type of 

 micrometer is the fact that the normal of the mirror is necessarily a pro- 

 longation of the axis of the screw, and also coincides very nearly with 

 the incident and reflected ray. There are thus no unknown angles be- 

 tween screw-axis and mirror-normal, and exchange of screws is no longer 

 necessary. When a mere comparison of screws is in question, it is 

 sufficient to reduce the play of image in the telescope to a small circle, 

 and this may be done in a few minutes. 



Auxiliary Apparatus for facilitating Adjustment of the Micro- 

 scope. "f — The purpose of this apparatus is to reduce the familiar 



* Amer. Journ. Sci., xxxv. (1913) pp. 267-9 (2 figs.). 



t Zeitschr. wiss. Mikrosk., xxix. (1912) pp. 193-4 (1 fig.). 



