HIGH-POWER OR CRITICAL PHOTO-MICROGRAPHY 137 



exposures go wrong when he least expects it, entirely owing — or anyhow very 

 largely — to his oxygen being exceedingly impure. To prevent any accident of this 

 kind, at Erin's oxygen works the oxygen is tested almost hourly, and users need 

 have but little fear that it is absolutely of the finest quality, which cannot be said 

 of the producers at cheap rates, for the writer has heard of some instances where it 

 was so very largely adulterated as to be next to useless. Another source of trouble 

 will come from a blackening of the lime during the exposure, which lessens the 

 brilliancy perhaps some 20 per cent. The firm to whom we have alluded have made 

 this an especial subject of study, and distinctly traced it to impurities in the coal- 

 gas, which when compressed have some ill-defined chemical action on the steel of 

 the tube itself After much consideration and experiment they have found that it 

 can be very largely — if not entirely — cured by coating the inside of the coal-gas 

 cylinder with a dressing which, whilst preventing the gas attacking the metal, in no 

 way alters its chemical properties. We have had our own coal-gas cylinder dressed 

 this way, and so far have found it very successful. 



[It is interesting now to take the same diatom with a half-inch N. A. '65, using a 

 high eye-piece or camera length to get the same magnification. It will be found that 

 although the pictures are of the same size, one reveals the dots and the other does not, 

 or if it does at all, most imperfectly. This is an illustration of the value of numerical 

 aperture, and has been alluded to above. See Fig. 10, Plate III.] 



Navicula rhomboides, Fig. 8, Plate IV. This specimen is a difficult one. 

 No oblique light is necessary and the " iris " must not be too much closed. Owing to 

 the specimen rarely being flat, focussing is extremely difficult and it is not often a 

 photograph can be taken with the dots equally sharp all over the ivhole picture. 

 Like the pleurosigma angulatum, this specimen is best photographed up to 1000, and 

 then re-enlarged from the primary negative. 



Coscinodiscus asteroirphalus, Fig. 6, Plate IV., selecting, of course, the fine 

 secondary markings, is also a specimen which requires considerable patience. These 

 require a faint trace of oblique light, and in the Zeiss pattern microscope that is 

 easily effected by turning the screw of the oblique-light-motion just a touch. In other 

 microscopes the auxiliary condenser and the light will require setting obliquely to a 

 small amount, or if using a mirror to reflect the light up the microscope it will have 

 to be somewhat obliquely placed. Focussing becomes now quite an art, and the 

 operator must not be disappointed if he has to take several negatives to get one 

 really well in focus. The diaphragm will require careful adjustment for this object, 

 but the Davis' must be opened wide. Exposure will here vary greatly according 



s 



