556 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The new method consisted in the replacement of the water in the 

 immersion system by cedar oil which is placed between the front surface 

 of the object-glass and the upper surface of the cover-glass of the 

 mounted object. The oil has the same refractive and dispersive power 

 as crown glass, and therefore the correction collar, though a refine- 

 ment having value still, was no longer inevitable. 



The construction of a combination of lenses which would satisfy 

 these conditions was earnestly desired and ultimately urged by Mr. 

 Stephenson upon Prof. Abbe ; and eventually the long series of re- 

 searches and experiments so efficiently detailed in the book we are 

 considering led to the formation of new vitreous compounds making 

 possible the large numerical apertures and almost perfect corrections of 

 an entirely new series of lens-combinations now known as " apochro- 

 matic " — opening a new era in Microscopy. 



This glass is now so generally used in all high class optical work 

 throughout the world, that a book like this giving authoritatively much 

 that it is of the greatest value to know concerning its construction and 

 its optical and mechanical properties, is a boon to all working opticians, 

 and a service rendered to mathematicians and physicists. 



Many pages are given to the consideration of the optical properties 

 of the glasses, and to the manner in which the perfection of optical 

 systems is secured by the utilisation of the special properties of the 

 glasses. Almost equally interesting is the discussion of the mechanical 

 properties of these vitreous combinations, which are carefully recorded and 

 explained. 



Much space is devoted to quite another feature which these glasses 

 in a marked degree are distinguished by, which is their endurance and 

 behaviour under varying thermal conditions. One important matter 

 especially to the employment of these compounds for optical lenses 

 and especially for the lenses of Microscopes, is the manner in which 

 lenticular surfaces made of the compounds are susceptible to tarnish 

 and inimical changes when exposed to varying conditions of atmosphere. 



From this book it is manifest what great advances can be made by 

 steady purpose in investigation and enterprising experiment. The ad- 

 vancement of practical optics by the devising of these vitreous com- 

 pounds has been very great, and as a side-issue it is not unimportant 

 that the discovery of the combinations has shifted the centre of the 

 world's optical work from England to Germany. The annual inflow 

 into England alone of optical instruments from Germany represents, 

 relatively, a new item and one of immense financial importance. But 

 it is not to be supposed that all that can be done has been done. May 

 we not hope that the enterprise of English opticians will lead them 

 to make effort, so that what is still attainable in the yet further ad- 

 vancement of the " metals " out of which lenses may be rendered still 

 more efficient, shall if possible be secured ? 



