92 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
THE IMMERSION METHOD. 
This method is based on the principles of oblique illumination outlined 
above and was apparently first described by O.Maschke in 1871, and again 
by him in greater detail and with the correct explanationin 1 880.* Maschke 
used and described the effects not only of oblique but also of central illumi- 
nation. To obtain oblique illumination, he either shifted the substage 
diafram of the Abbe condenser or shut off half the field by means of a black 
opaque strip placed between the objective and the grain under observation. 
In his description he includes a list of refractive liquids suitable for use with 
natural minerals. He used and observed the white-line effect, now usually 
known as the Becke line. 
In 1 880 Thouletf suggested the Thoulet solution as an appropriate re- 
fractive index liquid which can be diluted with water and a series of refrac- 
tive liquids thus prepared. His method for determining the relative 
refractive index of the mineral consisted in noting its disappearance in the 
liquid (absence of relief and of the shagreened surface) and did not furnish 
results of a high order of accuracy. 
In 1884 and 1885, C. Christiansen^ in considering the cause of white 
pigments, came to the conclusion that they consist of colorless bodies in a 
finely divided state, just as powered transparent glass appears white. He 
found that on immersing a quantity of powdered glass in a glass trough with 
transparent sides and filled with a suitable liquid, not only did the white color 
disappear, but the emergent transmitted light was practically monochro- 
matic and of the color for which both glass grains and liquid had the same 
refractive index, the remaining light having been scattered, by refraction 
and total reflection, at the surfaces of the innumerable glass particles 
immersed in the liquid. By changing the composition of the liquid slightly 
or by heating it, he was able to obtain monochromatic light of different 
wave-lengths. He found that if such a mixture of glass grains and refrac- 
tive liquid be placed in a hollow glass prism and then observed in sodium 
light, a sharp sodium line was visible and from it the refractive index of the 
mixture could be determined. After the grains of glass had settled to the 
bottom the clear liquid above had a different refractive index from that of 
the mixture. If the volume of the powder be T'i, that of the liquid V*, 
Vi-f- Fj that of the mixture, and n\, wj and N the corresponding refractive 
indices for sodium light, then the relation obtains 
(Vi+VdN-Vim+Vim (i) 
Theoretically this law of Christiansen states that the time in which light 
passes through the mixture, is equal to the sum of the times in which it 
passes through the components. This equation may also be written in the 
form 
and agrees accordingly with Gladstone's law of the refractive equivalent. 
If the compositon of the liquid is changed slightly, then 
\ (2) 
. Ann.. US, 565-368. 1871; WiedemantTt Ann.. II, 722-734. 1880. 
tBull. Soc. Miner, i, 62-68. 1880. 
iUntersuchung Obr d. optbchen Eigeiuchaften von fein vertciltcn Kdrpern, Wiedemann's Ann. d. 
Physik. 23, 198. 1884; 34, 439. 1885. 
