Barker — On Microscopic lUuminatioyi. 
7 
summer temperature, I now come to the evidence of a low winter 
temperature, Disodic sulphate or sulphate of soda is produced na- 
turally by double decomposition of certain salts in solution at a high 
temperature, or at very low temperatures. In the memoir of Mr. 
O'Eeilly and myself, already referred to, we have discussed very fully 
the formation of sulphate of soda in nature, and shown, as I believe, 
that the sulphate of soda of the beds of the Jarama could only have 
been formed by the natural decomposition of sulphate of magnesia and 
common salt in solution, or of sulphate of lime and common salt under 
the same circumstances. The presence of abundance of gypsum, and 
of the combination of the sulphates of lime and soda in glauberite, 
speaks strongly in favour of gypsum being the source, in part at least, 
of the sulphate of soda. The sulphate of soda beds and lakes of the 
steppes of the Aral Sea, described by Herr ^J'oschel, which are clearly 
the result of saline decompositions at low temperatures, afford a com- 
plete key to the formation of the sulphate of soda of the Jarama, In 
fact, the circumstances are so similar, that the only thing wanting in 
the Aral steppes to produce beds of thenardite, similar to those of Spain, 
is a temperature suf3S.ciently high to raise the solution of sulphate of 
soda in the lakes and ponds above 35° Cent. Such a temperature would 
be produced, if the excentricity of the earth's orbit were increased to 
what it was 200,000 or 210,000 years ago. 
Even though further investigations should establish that the depo- 
sits of the valley of the Jarama are older than the glacial drift, they 
will still be evidence of the prevalence of extreme temperatures, such 
as characterized the period of the glacial drift at another and an earlier 
period. Indeed, if, as some think, the glacial drift should be assigned 
to the last astronomical period of great excentricity, 200,000 years ago, 
and that the Jarama beds are older than the sands and clays near 
Madrid, in which the bones of an elephant were found, and which are 
probably contemporaneous with the drift, the thenardite of the Jarama 
might with great probability be assigned to the earlier cold period, 
750,000 years ago. If this were so, the interest of these deposits 
would be still greater, because for the first time we should have an 
absolute standard for measuring geological time. In any case the 
Jarama beds are deserving of the attention of geologists in connexion 
with the question of the climate of Europe at the beginning of the 
present epoch. 
III. — On the Illumination of Microscopic Objects, EyJoHN Barker, 
M, D. (Plates I., II., III.) 
[Read January 10, 1870.] 
The Academy are, doubtless, aware that one of the most important 
improvements of late years in object glasses of high powers has been 
the immersion of the object glass of a particular construction into a 
