154 BACTERIOLOGY. 



plates, and obtained the illumination by means of a small incan- 

 descent lamp, which gave a strong, white light, with three or 

 four Bunsen elements. In another respect Hauser's results were 

 of practical value. The preparations to be photographed were 

 stained brown as recommended by Koch, but they were mounted in 

 the ordinary way in Canada balsam. The objection to the mounting 

 medium most commonly employed was thus set aside. The prevalent 

 idea, however, that the preparations must be stained brown was still 

 a formidable obstacle, and the way out of this difficulty was clearly 

 shown by Van Ermengem's photographs. These were pictures of 

 comma-bacilli which had been stained with fuchsine and methyl 

 violet. These photographs afforded the first practical illustration 

 of the value of isochromatic plates in micro-photography which had 

 been previously noted by Van Ermengem in 1884, and their intro- 

 duction marks a distinct era in the progress of micro-photography. 



A .short explanation may be given of what is meant by isochro- 

 matic, or what have been more properly termed orthochromatic dry 

 platss. The difficulties encountered in photographing certain stained 

 preparations have been mentioned. It is a familiar fact that in 

 portraits, blue or violet comes out almost or quite white, while 

 other colours, such as yellow, are represented by a sombre shade 

 or perhaps black. This failure in correctly translating colours is 

 explained by the want of equality between the strength of the . 

 chemical and luminous rays. If the rays of the spectrum are pro- 

 jected upon a photographically sensitive surface, the greatest effect 

 is found to take place at the violet end. In other words, the violet 

 and blue rays are more actinic or chemically powerful, while the 

 yellow and orange have scarcely any effect. The dyes employed in 

 staining give corresponding results : blue and violet give but a faint 

 impression, yellow and orange a black picture. These results are 

 most clearly demonstrated in a photograph of an oil painting taken 

 in the ordinary way ; and they led to experiments being made which 

 have resulted in orthochromatic photography. 



The effect of interposing coloured glasses has already been 

 referred to. It was found later that, if plates were coloured yellow, 

 e.g., with turmeric, the blue and violet rays were intercepted, and 

 their actinism reduced. In 1881, Tailfer and Clayton produced the 

 so-called isochi'omatic plates. The emulsion of bromide of silver 

 and gelatine was stained with eosin, and it was claimed that colours 

 would be represented with their true relative intensity. Chlorophyll 

 and other stains have been tried, and by such methods the ordinary 

 gelatine dry plates can be so treated that they will reproduce 



