724 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



devoted to the question of the limit of resolving power, and the problems 

 especially associated with high magnification. 



Very remarkable is the way in which the whole subject is illustrated 

 by experiments, and experiments conducted by means, for the most 

 part, of apparatus of extraordinary simplicity. The writer's resource 

 in this direction may be illustrated by his discussion of subjective 

 colours, which is based, in fact, upon a coloured plate facing page 20. 

 Excellently coloured diagrams accompany the text and make the mean- 

 ing clear to the eye, and by an ingenious use of the tissue paper em- 

 ployed to protect the plate from contact with the printed page opposite 

 to it, the effect of letting a top light in to the stage of the Microscope 

 is most strikingly and successfully imitated. The work is spoken of in 

 the preface as a task which has occupied the author for many yea s, 

 and it may be admitted that the unusual amount of original work of 

 which the reader finds traces on almost every page, seems to warrant this 

 statement being taken au pied de la lettre. 



The book is admirably printed and illustrated, so that its publishers 

 may fairly claim to have placed the work before their readers in the best 

 possible form. 



Note on Sir A. E. Wright's Resolving Limit.* — Mr. Nelson writes : 

 " The wave-length selected by Sir A. E. Wright in his examples for 

 ultimate resolution is • 6 /x ; this lies on the red side of the sodium or 

 D line, whereas the one commonly used is that of maximum visual 

 intensity, which is situated on the blue side of the D line, about one-third 

 of the distance between D and E. 



" It is therefore not possible to compare his results with those in the 

 table at the end of this Journal, or with those in my table on p. 529 of 



this Journal (1906), before bringing them to terms of the same denomin- 

 ation. It will be noticed that the value of c adopted in the ' Principles 

 of Microscopy ' is 1-2. 



" The values for objectives with N. A.'s other than those in this table 

 may be found by inspection, by halving or doubling the values there 



* Principles of Microscopy, 1906, by Sir A. E. Wright, p. 231. 



