606 SUMMARY OF CURRENT RESEARCHES RELATING TO 



objective is inapplicable for any but opaque objects lighted in front, 

 it being difficult to secure approximately even illumination by this 

 means of transparent objects lighted from behind. 



" In the case of photographic objectives — I have used these of from 

 2-in. to 6-in. focus — an eccentric diaphragm of thin blackened brass can 

 be placed against the diaphragm between the combinations, and this 

 is certainly the better place for the screen. I have used a 2-in. focus 

 Ballmeyer portrait combination (which is specially good for this work), 

 a 4|-in. Unar, and a 6-in. Goerz. 



" Of course, for low magnifications of a rough object it is advisable 

 to work with a low power — the 4|-in. focus lens was used for most of 

 my earlier work — but for larger objects a 6-in. lens would define 

 better. The little Dallmeyer is a marvel for definition and flatness 

 over the small field used. I have had extra tubes made for my 

 Microscope (a Yan Heurck by Watson) to take the place of the lower 

 rackwork tube and the upper sliding tube, which carry the photographic- 

 lenses — the Dallmeyer at the bottom of the draw-tube, the Unar (for 

 Avhich I had a new mount made so as to get it inside the tube) about 

 l;j in. down the tube from the top, and the Goerz outside at the 

 eye-piece end of the Microscope. These adaptations enable the lenses 

 to be carried at the suitable distance from the object on the stage, 

 and allow sufficient rackwork for focusing. As the major conjugate 

 focus of the objective is used in Microscope work, all non-symmetrical 

 photographic lenses, such as ' Unars,' ' Stigmatics,' and portrait com- 

 binations, should be reversed on the Microscope (the front combination 

 being presented to the plate) to enable them to perform at their best. 



" When the distance is too great (as it will be in low-power work) for 

 the hand to reach the focusing pinion, I use a Hooke's universal joint 

 focusing rod, but for the higher powers I have a long rod on the other 

 side (the right) with a pulley- wheel near the end, over which and the 

 fine-adjustment screw-head runs a cotton-thread loop, which is quite 

 effective even with a ^-objective. 



" In photographing transparent objects, and for high-power work, I 

 use light of varying obliquity for the two halves of the plate, and the 

 getting the two halves into proper position on the plate is a rather com- 

 plicated business, as I use two different methods of obtaining oblique 

 light (each of which necessitates a different position of the plate in the 

 slide), and sometimes dispense with the eye-piece (which requires the 

 reverse position of the plate to that when the eye-piece is used). I have 

 to keep a memorandum sheet by me to prevent my making mistakes 

 when at work. For high powers, less obliquity of the light is necessary 

 than for medium or low powers, and in this case the condenser may be 

 moved laterally, but for medium and low powers an eccentric diaphragm 

 in the revolving cell under the condenser is a convenient mode. I have 

 had five diaphragm-carriers made to fit the revolving cell under the sub- 

 stage condenser, with apertures at different distances from the centre — 

 5, 1, 1^, 2, and 3 mm. Into these apertures drop plates, ten in number, 

 with openings from lf-20 mm. Thus there are fifty changes of aper- 

 ture and distance from centre, which will meet all demands for high- 

 and low-power work. 



