PROCEEDINGS OF THE SOCIETY. 609 



Mr- C. L. Curties exhibited a new 1/2 in. apochromatic objective by 

 Zeiss, with a numerical aperture of 0*60. This was only the second of 

 its kind which had reached this country, the other being in the hands 

 of Mr. Nelson, who spoke very highly in praise of its performance. It 

 was shown with an achromatic condenser designed by Prof. Abbe, who, 

 he believed had been converted to adopting achromatism in the 

 illuminator. 



Mr. J. Mayall, jnnr., said that it was upon the occasion of his visit 

 to Jena that he brought the attention of Prof. Abbe to bear upon the 

 subject of the achromatic condenser, and having shown him the advantage 

 obtained by its use he was in the end convinced of its value. The 

 instrument used by Mr. Curties was an embodiment of the idea then dis- 

 cussed by Prof. Abbe, the first ones made being intended for use in photo- 

 micrography. It had an aperture very close upon unity, so that it had 

 the maximum required for dry lenses. There was also an arrange- 

 ment by which if it was required for use with other powers the front 

 combination could be removed, and a larger field by that means obtained. 

 There was one point in connection with the use of these condensers, 

 which it seemed, from the work sometimes exhibited, needed to be rather 

 plainly dwelt upon : this was that the light should always be accurately 

 focused upon the plane of the object, otherwise the best results could 

 not be obtained. There were, as they knew, both thin plates and thick 

 plates upon which objects were mounted, and when an achromatic 

 condenser was made for use with a thin plate, it was unfair to use it 

 upon a thick one ; the proper thickness must be considered if the best 

 effect was desired, and it was very important, therefore, to note that the 

 focusing should be accxu'ately centered upon the plane of the object. 

 The thickness could be altered, sometimes, by turning the slide over, or 

 by putting additional pieces of thin glass temporarily above or below. 

 Some manipulators had adopted that plan, but others did not seem suffi- 

 ciently aware that the object was to give a very perfect image of the 

 lamp-flame. 



The President inquired whether these new condensers had a large 

 head, or were coned down. 



Mr. Mayall said that with so large an aperture as N.A. 1, the 

 condenser had to come up almost in contact, and being designed to work 

 with both medium and high powers the lenses had to be of large size. 



The President thought it would in this way be only applicable to 

 mounted objects, and not to live ones, especially as they wanted some- 

 times to reverse them. 



Mr. Mayall thought this would apply to all condensers, but would 

 depend upon the lenses in some degree. They had been very successful 

 in Jena in obtaining long working distances for their objectives, one of 

 1/8 in. used by Dr. Dallinger had two or three times the working 

 distance of those ordinarily made. This was due, no doubt, to the new 

 glass, and also to the great care taken in the coning of the lenses as 

 they lay one above the other. 



Mr. Western exhibited a species of Asplanchna, which, he said, the 

 President had been good enough to identify for him. 



The President said that when Mr. Western showed him these 

 specimens he recognized them at once as old friends from America, like 



