Mr. Highley's Condenser. 369 



If a student had a complete set of illuminating apparatus, 

 including Powell and Lealand's large angled condenser, the 

 parabolic illuminator, etc., etc., we do not say that for some 

 special objects he would not avail himself of these contrivances 

 in preference to Mr. Highley's ; but the latter has the great 

 practical advantage of being adapted to a wider range of 

 power and purposes, and is thus calculated to save an immense 

 deal of trouble and loss of time in shifting apparatus. 



The distance of the condenser from the object is regulated 

 in the best pattern of Mr. Highley's condenser by a rack ad- 

 justment • and in addition to the set of stops in the circular 

 diaphragm, he has introduced a double shutter on the plan of 

 Mr. Collins' clever adjustable diaphragm, which appears, as we 

 before stated, to be a revival of an arrangement of Dolland, 

 for which Mr. Collins deserves much credit. 



This shutter, by varying the size of the aperture, is 

 intended to correspond in action with the series of round holes 

 in the diaphragm of an ordinary condenser. To some extent 

 it does so perfectly, and it will be found very useful ; but a 

 small stop only admitting central rays in the ordinary pattern 

 appears to us preferable on some occasions. The abruptly- 

 rounded form of the bull's-eye in Mr. Highley's contrivance is 

 not favourable for the transmission of a small pencil of central 

 rays only ; and when trying to make it answer this purpose, 

 we have not succeeded in avoiding a certain amount of glare. 



The new condenser requires a little practice, and some 

 deviation from the mode of using the old condenser, to work 

 well. Thus we have obtained the best definition of the Podura 

 scale by using a central stop, and partially closing the shutters 

 of the diaphragm. A stop which admits light from a half- 

 circle of the margin of the lenses only, is very useful for many 

 purposes, and as all the stops are large it is often desirable to 

 try the effect of throwing them out of centre so as only to use 

 portions of them. 



We feel convinced that microscopists will appreciate the new 

 condenser, and it will save "many a student from the necessity 

 of purchasing several distinct pieces of apparatus, costing in 

 the aggregate a great deal more money than the price of Mr. 

 Highley's instrument. 



VOL. VIII. NO. V. B B 



