266 ROOD; ON THE PRACTICAL APPLICATION OF 



scope and treated like a microscopic object, the magnifying 

 power varying from five to twenty diameters. If the prints 

 are to be on paper, it will be found that a more liberal use of 

 nitrate of silver and chloride of gold than is generally recom- 

 mended makes success easy. 



Magnifying poivers employed. — To produce enlarged images, 

 the objectives, as is wxU known, may be used alone or in con- 

 nection Avith an eye-piece. In the former case, with proper 

 illumination, sharp images are produced when the distance 

 between the object (on the stage) and the ground glass is as 

 great as five feet. With this distance the 



1-inch enlarges 65 diameters. 



i „ >, 460 



In using the objectives in this way the screw collar is set 

 after the microscope is connected with the camera. 



For more highly enlarged images, it is best to add the long 

 eye-piece, as has been practised by some experimenters. The 

 adjustment of the screw collar can then be very nearly com- 

 pleted, before the microscope is connected with the camera, 

 which is a great saving of time ; it will, of course, fall nearer 

 the mark "uncovered^^ than in the first case. However per- 

 fectly this operation may be performed in either instance, 

 allowance must still be made for the actinic focus. By vary- 

 ing the distance between the eye-piece and the ground glass, 

 diflerent degrees of enlargement are obtained. When the 

 long or 2-incli eye-piece is used, the distance from the 

 object-slide to the eje-lens being twelve inches, from the latter 

 to the ground glass thirty-four inches, then 



1-inch enlarges 160 diameters. 



i ,, 3> 1300 



Powers obtained in this way with the two latter objectives 

 have been used by me with advantage. 



Thus with the 4th, 113° aperture, the Wollaston doublet of 

 44° aperture, having a central stop, being used as a condenser, 

 I obtained sharp negatives of P. angulatum magnified 1300 

 diameters, with well-defined hexagonal markings similar to 

 those obtained by Wenham with a Vrth of 130° aperture. 

 Portions of the negative bore a photographic enlargement of 

 ten diameters. Mr. Wenham announces''^ that he has dis- 

 covered, bj' the use of a J^r*^ of large aperture, made by 

 himself, that the markings on this object and on some others 

 are really due to spherical particles of quartz, which can be 

 * 'Quart. Journal of jMic. Science,' No, xxxi, p. 145. 



