146 



THE AMERICAN MONTHLY 



[August, 



ted light required first that the source 

 of Hght should be pictured by the con- 

 denser exactly in the plane of the ob- 

 ject, the object then served to interrupt 

 the image of the source of light. The 

 observer had simply to airange the 

 lamp, condenser, and diaphragms so 

 as to produce the most perfect image 

 of the source of light of the required 

 size in the plane of the object, the 

 objective would then have fair play. 

 The size of the image of the lamp 

 flame could be controlled by dis- 

 tancing the lamp, as illustrated by a 

 diagram. There really was no other 

 secret in the matter. With the in- 

 candescent lamp the image produced 

 by the condenser represented the 

 mere carbon thread, on vs^hich no ob- 

 ject could be seen projected ; in order 

 to obtain some extent of brightly lu- 

 minous field the condenser must be 

 put out of focus ; then the intensity of 

 the light was so reduced that the ob- 

 server would simply discard the in- 

 candescence, finding it far less ser- 

 viceable than a shilling parafiin lamp. 

 He had himself suggested the con- 

 struction of a lamp for use with the 

 microscope, in which the flame could 

 be brought very low down near the 

 table. The glass of the chimney con- 

 sisted of an ordinary 3 by i slip, and 

 so could be cleaned or replaced read- 

 ily. The chimney was of metal, and 

 he preferred it black inside ; he want- 

 ed the image of the flame on a black 

 background. The bull's-eye of the 

 ordinary consti'uction gave but a small 

 disc of clear light ; he had therefore 

 engaged Messrs. Swift to construct a 

 condenser for the lamp on the Hers- 

 chelian form, consisting of a menis- 

 cus and a bi-convex, which gave 

 a much larger and clearer disc of 

 light, as shown by diagram. He did 

 not advise the use of the bull's-eye for 

 direct transmitted light ; it was pref- 

 erable to point the microscope direct 

 to the flame, or to use the plane mir- 

 ror. The bull's-eye was useful for ob- 

 lique light, especially when a double- 

 slot diaphragm was required. It was 

 very important for dark-ground illu- 



mination, and with the Lieberkuhn. 

 Oblique light with single-slot dia- 

 phragm he obtained better without 

 bull's-eye. In treating of the use of 

 condensers he could not too strongly 

 dwell upon the importance of pro- 

 portioning their aperture and power 

 to those of the objectives. In his 

 judgment no condenser had yet been 

 devised so effective as Powell and 

 Lealand's for all powers beyond ^. 

 It had an aperture of about 170°, and 

 a focus long enough to go through 

 any ordinary slip, and was provided 

 with a beavitifully centred disc of 

 graduated diaphragms, and with slots 

 and central stops of admirable con- 

 struction. Whilst giving it the palm 

 over every other form of achromatic 

 condenser, he was alive to the fact of 

 its being costly. It was so w^ell made 

 that it must be costly. Its conve- 

 nience was so great that he could 

 hardly suppose that any one who had 

 once gained experience of its utility 

 would consent to use high powers 

 without it. He regarded it as the 

 most useful apparatus in his collec- 

 tion. Messrs. Swift have produced 

 a condenser of 145° degrees aperture, 

 which, by removing the front lens, 

 could be reduced to a much lower 

 aperture suitable for low powers ; it 

 had a pivoted disc to carry the dia- 

 phragms. The microscopist should 

 possess both these condensers to en- 

 able him to do critical work with 

 high and low powers. With Messrs. 

 Swift's condenser it was advisable to 

 have an adapter to fit beneath and 

 carry three concentric rotating rings, 

 which were specially convenient for 

 experimenting with central stops, or 

 for zonal illumination. The super- 

 position of two or three discs differ- 

 ently cut gave a great range of effects 

 of light. They might also be used 

 for selenites and mica, and would be 

 found of great service in this combi- 

 nation. He entirely condemned the 

 use of paraboloids for dark-ground 

 illutnination. Propeidy-adjusted cen- 

 tral stops with the condenser would 

 give by far the best dark-ground illu- 



