158 THE MICROSCOPE. 



of the microscope altogether. The time of producing the 

 picture varies from five to twenty seconds, with the strength 

 of the daylight. A camphine lamp, light Cannel coal-gas, 

 or the lime-light, will enable a good manipulator to pro- 

 duce pictures nearly equal to those produced by sun-light. 

 Collodion offers the best medium, as a strong negative can 

 be made to produce any number of printed positives. 



The light is transmitted from the mirror through the 

 object and lenses, and brought to a focus on the ground- 

 glass, or prepared surface of collodion, in the usual manner. 

 Care must be taken not to use the burning focus of the 

 lenses. The gas microscope may be used to make an 

 enlarged copy of an object, it is only necessary to pin up 

 against the screen a piece of prepared calotype paper to 

 receive the reflected image. Mr. Wenham gives direc- 

 tions for improving "microscopic photography " in the 

 Quarterly Journal of Microscopical Science for January, 

 1855. In this paper he has shovn how to insure quick 

 and accurate focussing ; or, in other words, the making of 

 the actinic and visual foci of the objective coincident. The 

 simplest and cheapest way of producing coincidence is to 

 screw a biconvex lens into the place of the back-stop of 

 the object-glass, which thus acts as part of its optical com- 

 bination. An ordinary spectacle lens, carefully centred 

 and turned down to the required size, answers the purpose 

 exceedingly well. 



An excellent method has been proposed and adopted by 

 Mr. Wenham, for exhibiting the form of certain very 

 minute markings upon objects. A negative photographic 

 impression of the object is first taken on collodion, in the 

 ordinary way, with the highest power of the microscope 

 that can be used. After this has been properly fixed, it is 

 placed in the sliding frame of an ordinary camera, and the 

 frame end of the latter adjusted into an opening cut in 

 the shutter of a perfectly dark room. Parallel rays of 

 sunlight are then thrown through the picture by means 

 of a flat piece of looking-glass fixed outside the shutter at 

 euch an angle as to catch and reflect the rays through the 

 camera. A screen standing in the room, opposite the lens 

 of the camera, will now receive an image, exactly as from 

 * magic lantern, and the size of the image will be proper, 



