44:2 PROCEEDINGS OF THE SOCIETY. 



placed, the cone of rays was far too small to be aflfected by tbe size of 

 apertures adopted, passing, in fact, completely within the apertures. 

 He might apply the same remarks to the action of the calotte dia- 

 phragms, which he regarded as based on a wrong conception of the 

 action of diaphragms. He could not commend the iris diaphragm on 

 the ground that it required a special fitting, and could rarely be used 

 near enough to the slide. 



Mr. J. Mayall, jun., said there was another purpose in the appli- 

 cation of diaphragms, not touched upon in Mr. Ingpen's remarks, 

 namely, the cutting off different portions of the illuminating pencil. 

 The mere cutting down the diameter was the main object of the 

 wheel of apertures in common use, and of the cylinder diaphragms 

 referred to, but Dr. Anthony's diaphragm was intended to supplement 

 the action of the strictly central apertm-e by a series that could be 

 easily applied to cut off more or less of the beam after all had been 

 done that was possible in modifying the light with the central aper- 

 tures, — to use a phrase of Dr. Anthony's, " to give the finishing touch 

 to the illumination." Regarding the ca?o//e diaphragm, its ai)plication, 

 as a diaphragm alone, immediately beneath the slide, was due to Mr. 

 Zeiss, who was hardly likely to have adopted it unless he had found 

 it effective. The still more recent application of it above the con- 

 denser must be regarded as a step in advance. Mr. Bulloch, of 

 Chicago, aj)peared to be one of the earliest to see that the diaphragms 

 beneath the optical combination in Gillett's condenser, might be ad- 

 vantageously applied above the lenses, where the cone of rays is so 

 short and of such great angular extension that every variation in size 

 or shape in the apertures of the calotte would be effective. Mr. Swift 

 had also adopted the calotte in connection with the achromatic con- 

 denser. The iris diaphragm was effective for low powers, especially 

 when mounted to fit in the stage itself, as adopted by Messrs. Eoss ; 

 but he had not been satisfied with it in connection with the achromatic 

 condenser. He believed there were difficulties in the construction 

 which rendered it almost impossible to close the aperture with suffi- 

 ciently accurate centering to be of real service with the condenser. 

 In conclusion, Mr. Mayall said that the great number of devices 

 that had been brought forward in recent years to cut off portions of 

 the illuminating pencil independently of the mere reduction of the 

 cone by strictly central apertui'es, proved conclusively that a need 

 was felt in that direction. 



Mr. Beck said that though there might be differences of opinion 

 as to what was the most valuable kind, he thought no one would 

 dispute the great importance of a good diaphragm, which was of 

 extreme value in rendering visible portions of an object which other- 

 Avise could not be seen. 



Mr. Ingpen said that his remarks were merely taking things as 

 they stood, and did not, of course, apply to the use of the calotte dia- 

 phragm with the achromatic condenser. The calotte diaphragm, as 

 drawn by Mr. Mayall, was very effective, but almost every effect could 

 be obtained by a very small number of stops with tolerably small 

 apertures. Professor Abbe had satisfied himself of this entirely. 



