492 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



15° of cold to 60° of heat. Another advantage of the lens is that on 

 account of the fact that the fluid is not dense, and the glass crystals are 

 thin, the whole lens combination through which the light penetrates is 

 very slight. 



Malassez, M. L. — Evaluation des distances fcco-faciales des Objectifs. 



[The substance of this treatise has already appeared in the Journal : 1905, 

 I p. 500, and 1906, pp. 362-3]. Comptcs Rendus, cxlii. (1906) pp. 926-8. 



(3) Illuminating- and other Apparatus. 



Zeiss' Centring Achromatic Condenser.* — In this condenser (fig. 

 57), N.A. 1*0, and equivalent focal length 14 mm., the stopping down 



Fig. 57. 



of the illuminating rays is effected by means of an iris diaphragm 

 situated between the lenses. This entails a full opening of the dia- 

 phragm of the illuminating apparatus. 



Spectrohelioscope (A. Sauve). — New Arrangement for procuring 

 a Monochromatic Image of a Light-Source (A. Nodon).| — Both A. 

 Sauve and A. Nodon, under the above dissimilar titles, have inde- 

 pendently studied the improvement of spectroheliographs for photo- 

 graphing the sun's image with light of definite wave-length. Both 

 authors have endeavoured to find means whereby the heavy spectrograph 

 itself should be at rest during use, and whereby all adjustment should 

 be attained by movement of the mirror which reflects the light on to the 

 slit. In Sauve's arrangement, the light which at first falls on the front 

 of the mirror is, after a series of reflections and refractions, made to fall 

 on the back of the same mirror before the final reflection into the 

 observation tube. 



Nodon attains the same object by the use of two simple mirrors 



* Carl Zeiss' Catalogue of Microscopes and Microscopical Accessories, 33rd ed. 

 1906, p. 31 (fig. 11). 



f Mem. della Soc. degli Spettroscopisti Italiani, xxxiiii. (1904) p. 54. See also 

 Comptes Rendus, cxli. (1905) p. 1010; and Zeit. f. Instr., xxvi. (1906) pp. 129-30. 



