720 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



white glass at d, d. The beam, now perfectly polarised, may be con- 

 verged or used parallel as desired. 

 The prisms and mirrors are firmly 

 mounted in a metal frame or box 

 which rotates in the bearings of the 

 supports e and /, rising from the 

 optical bars of the lantern, and thus, 

 as with the large Nicol or Foucault 

 prism, the plane of polarisation may 

 be turned to any angle with the ver- 

 tical. This compound polariser is 

 almost exactly the same length as a 

 Nicol of the same clear aperture, and 

 gives very nearly as much light. 



A very convenient size for the 

 polarisers is a 2-in. aperture ; but 

 they may be made 3 or 4 in. in dia- 

 meter if desired. For experiments 

 where parallel rays only are used, a 

 pair of these compound polarisers give 

 results not inferior to those obtained 

 with the Spottiswoode prisms used by 

 Prof. Tyndall. In general, however, 

 it is better to use as an analyser a 

 Nicol prism of 20 mm. aperture, as re- 

 commended by Wright, using the new 

 prism as a polariser only. 



(4) Photomicrography. 



Scott's Apparatus for Instanta- 

 neous Photomicrography.* — Mr. A. 

 C. Scott, of Ehode Island College, has 

 succeeded in obtaining instantaneous 

 photographs of living organisms. A 

 very powerful light is required, and 

 he gets this from an arc light of 

 2200 watts, which gives about 4000 

 candle-power. This light is placed 

 at a distance a little greater than the 

 focal length of the condensing lens, 

 so that the intensity of light upon the 

 object and objective is considerably 

 greater than would be the case with- 

 out the lens. Of course a different 

 position of the lens and light would 

 greatly magnify the intensity of the 

 light, but that is undesirable beyond 

 a certain limit, as the heat would be 

 detrimental to the Microscope objec- 

 tive. The important items of com- 



* Journ. App. Micr., March 1900, pp. 797-9 (4 figs.). 



