Progress of Invention, 469 



not lost to the same extent as with, lower power. They have, in 

 fact, nothing like so much to lose. The difference is in other 

 respects the same. 



The two forms of illuminator for high powers which we 

 have described, will work, as we have found, with a fifth and 

 "with a 1 -20th upon any object sufficiently flat, and possessing 

 sufficient reflecting power. Yery minute markings can be 

 seen; but it is far from easy, if possible, to avoid the appear- 

 ance of a certain milkiness in the field. Possibly — but we 

 cannot yet say — the plan adopted by Mr. Smith may be the 

 best, but the three plans will stand in successive positions of 

 complexity and price. Mr. Beck's is the simplest and cheapest. 

 Messrs. Powell and Lealand's is in some respects more complete 

 and more costly, and when these latter gentlemen produce, as 

 they intend doing, a more close imitation of Mr. Smith's, its 

 complexity will be greater and its price must be proportionately 

 higher. The subject is too new for us to be able to say more 

 than that these reflectors are highly important inventions, which 

 deserve the immediate attention of all careful microscopists. 



PROGRESS OF INVENTION. 



Improvement in the Process foe Obtaining Carbonate of 

 Soda. — The economic production of carbonate of soda is of con- 

 siderable importance to several of the arts and manufactures. The 

 process invented by Leblane for the purpose was so effective and 

 convenient, that it has remained in use with scarcely any modi- 

 fications for a long period. It consists in heating common salt with 

 sulphnric acid ; forming, by means of caustic lime, and carbonaceous 

 matters, a mixture of caustic soda and its carbonate, from the 

 sulphate which is obtained; dissolving these in water, to separate 

 them from the other substances with which they are associated ; 

 evaporating the solution, and changing the caustic soda of the 

 residual mass into a carbonate, by heating to a high temperature 

 with sawdust ; then dissolving out the salt, concentrating the solution, 

 and crystallizing. This process has now been simplified, by decom- 

 posing the sulphate of soda obtained from common salt with caustic 

 baryta, which throws down insoluble sulphate of barytes ; and 

 then changing the caustic soda which remains in solution into car- 

 bonate, in the ordinary way. It is probable that this method would 

 have come into use long since, had it not been found troublesome and 

 expensive to separate the sulphuric acid from sulphate of barytes, so 

 as to obtain economically the required caustic barytes. Mr. Hunter 

 has, however, overcome these difficulties, by acting on the sulphate 

 of barytes with lime, under a pressure much more considerable than 



