528 Notices respecting New Books. 



43. In our study of microscopic vision no consideration 

 has been given to the consequences of imperfections in the 

 mounting of the instrument, or in the objectives and eye- 

 pieces. To have entered on these branches of the subject 

 would have been to open new ground, and ground, which has 

 been rendered of less importance by the extraordinary per- 

 fection both in the stand and in the objectives which may be 

 secured by a careful selection from among the best that are 

 available. It is truly astonishing with what accuracy the 

 chromatic, and especially the even more important spherical 

 aberrations have been successfully corrected in some speci- 

 mens from anion o' the best objectives on sale. Objectives 

 are not unfrequently to be had which, when the draw-tube 

 is adjusted with sufficient care, will bear an eyepiece mag- 

 nifvino; 40 times without observable defect. 



However great their degree of perfection, it may be 

 carried one step further by a skilful use of monochromatic 

 light; whereby adjustments can be made by the observer 

 with a completeness which with light of mixed wave-lengths 

 is unattainable. This is indicated by theory, and abundantly 

 confirmed in practice. 



In whatever branch of microscopic work the observer may 

 be occupied he will find it of advantage to train himself both 

 in the intelligent manipulation of his instrument and in the 

 interpretation of results, by making a large body of experi- 

 ments such as those of which a few selected examples have 

 been described in the foregoing pages ; being careful at every 

 step to understand the reason for everything he does, and to 

 find out the cause of every efect he 'perceives. To enable him 

 to do this has been the object of the present memoir. 



^\ 



LII. Notices respecting New Books. 



Studies in Chemical Dynamics. By J. H. VAN" 't Hoff. Revised 

 and enlarged hy Dr. Ernst Cohen", translated by Thomas 

 Ewan, M.'Sc, Ph.D. Amsterdam : Frederick Muller and Co. ; 

 London : Williams and Norgate. 



THE present translation of Professor van 't Hoff's work will 

 come as a surprise to many English chemists who are not 

 aware of the magnitude and importance of his researches on some 

 fundamental problems of chemical dynamics. In Britain there are 

 but few workers in the same field, and, as the translator tells us, 

 scarcely any text-book in our language deals with the subject. The 

 reason is, we believe, not far to seek ; it is, however, a most 

 lamentable one. 



