( 170 ) 



The illumination of such a colorimeter is generally effected by 

 means of a mirror placed below the tubes, which reflects the light 

 from the sky. Owing to the clouds, this illumination may be very 

 irregular, therefore artificial light is preferable. Incandescent light 

 is very satisfactory. With artificial light, however, a mirror cannot 

 be used, as small displacements of the lamp greatly affect the illu- 

 mination of the two colorimeter tubes; instead of a mirror, a piece 

 of ground milky-glass is then employed. 



Above the tubes is placed the optical arrangement, which serves 

 to create a field of vision, which is divided into two parts, one of 

 which is illuminated by the rays, which have traversed the standard 

 tube and the other by those, which have traversed the measuring 

 tube. 



In principle, it is preferable to make both halves of the field of 

 vision exactly the same shape, as they are then observed under 

 exactly the same conditions. These conditions are not satisfied by 

 the prism-system of Lummer and Brodhun, which has been applied 

 by H. Krüss to the WoLFF-colorimeter '), The field of vision is here 

 a circle surrounded by a ring. This may, perhaps, partly explain 

 the less favorable report of the Photometer Committee of the Nether- 

 land Society of Gasmanufacturers ^). 



The prism-system of Fresnei., generally met with in colorimeters, 

 suffers from the drawback that it is liable to give way, when being 

 cleaned, and cannot then be again properly joined together. This 

 creates in the field a heavy black line of junction, which greatly 

 impedes an accurate observation. A prism made from milky -glass ') 

 is not advisable on account of the transparency which causes the 

 two halves to illuminate each other in the neighbourhood of the 

 line of junction. An equality of colour is then noticed before it is 

 really a fact. 



I use a prism of polished telescope-metal with angles of 45° 

 illuminated by two little mirrors also at angles of 45° placed above 

 the tubes. The line of junction is then hardly visible and the prism 

 is proof against the influences of a laboratory atmosphere. 



The apparatus is now used as follows : The standard tube and 

 the vessel A are provided with standard liquid, and fixed in such 

 a manner that the height indicated on the rod B really corresponds 

 with the position of the liquid in S. The standard liquid would 

 have to be more diluted than the solution to be measured. A known 



1) Zeitschrift fur Instrumentenkunde 14. 102. 



2) Report of the said committee 1803. 



3) Ostvvald I.e. p. 180. 



