60 THE VICTORIAN NATURALIST. 



matic condenser was mentioned as being of N.A. 1-36, but owing 

 to lack of correction only N.A. 5 was available for central illumina- 

 tion, the larger portion of the cone tending to fog the image if not 

 stopped out. The steps to set up the microscope for the pro- 

 duction of a critical image were then explained in detail, each 

 step being written out and numbered for after reference. The 

 ultimate aim was shown to be the production of an image of the 

 source of light, in which the object could be seen immersed, both 

 object and flame image being equally in sharp focus and centred. 

 As this might result in only a portion of the field being illuminated, 

 it was shown how, by using a condenser suitable for the objective, 

 or an adjustable one, or by using the bull's-eye condenser in con- 

 junction, this end could be secured where desirable. Stress was 

 laid on the points that the diaphragm apertures should only be 

 used for adjusting the illuminating cone to the angle suited to the 

 objective used, not for modifying its intensity, which should be 

 done by interposing blue glasses of suitable depth of tints, and 

 that only in the image of the source of light, or " flame image," 

 could a critical picture of the object be found. Then the speaker 

 alluded to the effects produced at the back of the objective when 

 viewed down the tube with ocular removed — effects which at once 

 afforded a means of seeing that adjustments were properly made 

 and that the cone of illumination was in proper relation to the 

 cone admitted by the objective. After showing how critical 

 pictures could be produced with dark field illumination, illumina- 

 tion with condenser alone, as also with condenser and mirror, the 

 objection sometimes raised that condenser illumination is un- 

 desirable for other purposes than resolving diatoms was criticised, 

 and the absurdity of attempting to use the high powers without 

 a condenser was insisted upon since the mirror could only pro- 

 duce a cone of about 30° angle, whereas an objective of 100° 

 requires about 75°. It was further argued from practical ex- 

 perience that while saving time — when once the process was 

 understood — not only diatoms but objects such as Protococcus, 

 Euglena, collared cells of sponges, sections of tissues, suctorial 

 tentacles of Acineta, and nuclei of Amcebse, were all better seen, 

 and their structural details more readily made out when examined 

 by proper combinations of objective and condenser. 



At the conclusion of a clear and incisive address the members 

 present examined two microscopes of Zeiss, both being provided 

 with D objectives and No. 2 oculars, one being set up with an 

 Abbe condenser for "critical" work, and the other with mirror only, 

 the object being in each case the fine hairs on the tongue of a 

 blowfly. This exhibit showed to those present the great superiority 

 of condenser illumination, since the admitted uniformity of the 

 glasses of this maker, and the well-known character of the object, 

 removed elements of doubt, and enabled all to come to a speedy 



