104 



It consists of a small iris-diaphragm fitting into the upper end of the tube 

 and a RAMSDEN eye-piece. This latter slides in a sleeve and is adjustable so 

 as to have the iris-opening in its focus. A glass disk with crossed lines lies 

 immediately below the iris-diaphragm. The crystal to be examined having been 

 accurately centred by means of the eye-piece and the other portions of the 

 image cut off by the iris-diaphragm, the eye-piece is then removed and for the 

 purpose of observing the axial image the analyzer put into its place, or an 

 analyzer, if otherwise fitted to the tube, put into operation. 



Price : Code-word : 

 No. 178: CZAPSKI's Iris-diaphragm ivith Eye-piece . . Marks 25. . Pentlra. 



Crystal Films and Plates for determining the character of double 

 refraction and the directions of extinction, 



Selenite and Mica Films. We supply regularly selenite films of four 

 different thicknesses which, placed between crossed Nicol prisms, show Red of 

 the I., II., III. and IV. Order; also four different mica films which are graduated 

 so as to produce differences of phase equal to x / 8 A, */4 ^> 3 /s ^ an( ^ Vz ^ 



These 8 films compose the sot of selenite and mica films originally proposed 

 by H. T. MOHL for the examination of plant cells in polarized light. If not 

 ordered otherwise, we issue the films mounted on paper disks, adapted for 

 Polarizer I in connection with the ABBE illuminating apparatus. 



Prices : Code-words : 



No. 179: MOHL's Set of Films Marks 75. . Peneque. 



Single films mounted on paper: 



180: Selenite films, Red of the I. Order, each . . j.- . Penetrar. 



other Orders . . 2.. 



Mica films, each 7.50. 



We also supply the same films in metal mounts either for putting into 

 the slide above the objective, as adapted for the Mineralogical Stands l b and IX b , 

 or for insertion between the eye-piece and analyzer on Stand VI b . 



Single films in metal mounts; 

 for Stands I b and IX b 



Prices: Code-word: 

 No. 181: Selenite films, Red of the I. Order, each . . Marks 4. . Penible. 



other Orders, . . 3. . 



Mica films, each 2.50. 



