752 PEOCEEDINGS OF THE SOCIETY. 



purposes of that evening's discussion, it was quite possible to accede to 

 Mr. Rheinberg's proposition. Then it had been said that the markings 

 shown in the photographs exhibited might be intercostal markings, but 

 that was a mistake. It was easy to distinguish intercostal markings 

 from objective features by varying the angle of the transmitted beam, 

 as, for example, by altering the iris diaphragm, and he could assure 

 them that he had not shown any intercostals that evening. 



Referring to Mr. Conrady's criticisms, and in particular to his refer- 

 ence to the abandonment by astronomers of the annular aperture, he 

 pointed out that there was a great difference in this respect between the 

 telescope and the Microscope, and that wholly different considerations 

 came into account when it was a question of applying the annular 

 aperture to the one or the other instrument. In the case of the tele- 

 scope, the object-glass was evenly illuminated all over by light received 

 in the form of plane wave-fronts from an object the absolute illumina- 

 tion of which the observer could in no way control. In the case of the 

 Microscope, a large proportion of the objects with which we deal do not 

 throw their light evenly upon the object-glass at all, but send it forward 

 in broken beams, which impinge at various points upon the face of the 

 objective. Hence, in the case of the Microscope, a stop affects different 

 parts of the picture in different ways, and may be used to darken the 

 field in contrast with a highly refractive object lying in the field, or, 

 vice versa, to darken the object in contrast with the field, and so, in a 

 great variety of ways, to render various features conspicuous, and so to 

 vary the character of the image in ways that are very helpful and highly 

 instructive. 



He hoped he had not been unfair to this portion of the subject, but 

 the annulus was merely the remaining part after the centre had been 

 stopped out, and he thought there could be no question as to what the 

 effect of this would be. Mr. Conrady seemed very much alarmed lest 

 it should produce intercostal effects, but, of course, the aperture must 

 be adjusted so as not to do this, and, though it was very easy not to 

 get them, it was quite easy to produce them if proper care was not taken. 



Mr. Gordon writes to the Editor asking leave to add the following 

 remarks upon a point in Mr. Conrady's criticism which escaped his 

 memory at the moment of replying at the Meeting. 



" Mr. Conrady has insisted on the distinction between ' resolution ' 

 in the proper sense of that word and the imaging of isolated objects, 

 and states that it has long been known that single objects could be ren- 

 dered visible, however small, if there is sufficient light available for the 

 purpose. He instances, in illustration of his point, the extreme case of 

 a star seen in a telescope or by the unaided eye. This statement, 

 although perfectly true of bright objects on a dark field, is quite irrele- 

 vant to the matter which I have been this evening discussing, that is to 

 say, dark objects on a bright field, and it would not be correct to say 

 that it has long been known that a dark object as small as a star can be 

 seen in the daylight sky ; it would not even be true to say that so small 

 an object could be so seen. Thus a telegraph wire projected against a 

 bright cloud becomes invisible at a very moderate distance, and when it 



