656 



XVII. — A New Semi-apocliromatic ^. 

 By Edward M. Nelson. 



{Read October 16, 1907.) 



During the recess a new objective, viz. a -^ of IST. A. • 74, com- 

 puted by Mr. Conradj'', has been brought out by Messrs. Watson 

 and Sons. The novelty of its construction gives it the altogether 

 abnormal working distance of 1 mm., an attribute that will 

 render this objective in many ways useful to biologists, medical 

 men, metallurgists, etc. 



Measurements show that its power and aperture are as stated. 

 Its performance upon the usual test objects is exceptionally good ; 

 the lenses are very well put together, and exhibit no signs of 

 excentricity. With the large W.x4.. of • 65, and F-line screen, 

 Grayson's 60,000 band was resolved ; this agrees with the table of 

 limits,* where 60,300 is the limit given for KA. 0-75. Sections 



Fig. 108. 



of animal and vegetable tissues, and entomological details were 

 brilliantly shown. The images of diatoms were particularly bright 

 and clear, those of Angulatum, Formosum, and Navicula rhoniboides, 

 were very sharp. Few lenses (apart from apochromats) have shown 

 •as well as this new -^- a balsam-mounted coarse Formosum, the most 

 severe test to which a dry lens such as this can be subjected. 



Bacteria (not that they are of much service as tests for micro- 

 scopical lenses.t but as several Fellows of this Society belong to 

 the medical profession, some among them might like to know the 



* See this Journal, 1906, p. 529. 



t Obviously it is impossible to form any judgment of the colour corrections of 

 objective from the inspection of a purple- or blue-stained bacterium. It would 

 seem that the only test that can he made with bacteria is the relatively unim- 

 portant one of flatness of field, which is done by comparing the sharpness of the 

 image of a bacterium at the edge with one in the centre of the field. If an object 

 at the edge of the field is out of focus when the centre is in precise focus, and 



