254 PEACTICAL PHOTO-MICKOGBAPHY 



mastered it is possible to produce photographs of living tissues 

 and organisms, or in an unaltered condition immediately after 

 death, which cannot be accomplished in any other way. 



It is in some respects unfortunate that the apparatus is 

 so expensive, as it of necessity limits its application, and ex- 

 cept only in scientific institutions few will be able to take 

 advantage of it. The further difficulty of obtaining fresh 

 preparations and having them mounted between the necessary 

 quartz slip and cover-glass presents an obstacle in many cases. 

 Preparations have to be prepared and dealt with immediately, 

 there being little possibility of leaving them over for any 

 lengthened period. 



In actual work the length of the spark between the cadmium 

 or magnesium electrodes should be kept as short as possible ; 

 what is usually referred to as a ' fat ' spark will be the most 

 satisfactory. It is permissible to use an induction coil giving 

 a larger spark than that mentioned ; but if so, the spark-gap 

 must be short, so that the spark itself becomes broader. The 

 spark spectrum of the electrodes projected into the apparatus 

 will then have greater intensity. The disintegration of the 

 electrodes takes place, of course, at their surface, so that the 

 shorter the spark-gap, having due regard to the necessity of 

 having an illuminant of sufficiently large area to fill the con- 

 denser, the better the result. 



The greatest practical difficulty is in focussing the objective 

 and the sub-stage condenser, whether dry or immersion. In 

 either case, the image as seen with a searcher eye-piece may 

 not give the best result in the photograph. As in photograph- 

 ing spectra, the best result is not always obtained when the 

 best visual image is seen on the focussing-screen ; hence it is 

 largely a question of trial and error. Alternatively, the searcher 

 eye-piece may be dispensed with and a series of exposures 

 made, with the fine-adjustment of the microscope so arranged 

 that successive planes at short distances apart are brought into 

 focus. A graduated milled head on the fine-adjustment is in 

 this case essential, and a succession of photographs would be 

 taken, the milled head being moved backwards or forwards by 

 one graduation only in each case. If this method is adopted 

 it is advisable to get an approximate image with the searcher 

 eye-piece and then to begin the series of trial exposures. The 



