104 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



object, e.g. on a spectrometer slit. The tube is engraved with the 

 mark oc and 1 : 1 in order to denote the necessary positions of O3. 

 The quartz double prism can be removed from its compartment P^ ; it 



Fig. 9. 



00 11 





"^0^" 



Fig. 10. 



has a dispersion of 894 /x/x, to 185 /x/x, about 10° 40'. The lens O3 is 

 an air-tight achromat of quartz rock-salt quartz. 



The objectives 0^ and 0.^ are also adjustable and have focal lengths 

 proportional to F : 4 and F": 6 respectively. The free distance of both 

 objectives is 30 mm. 



