ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 581 



factnre are not given in English treatises. The collodion process (wet 

 plate) is used, or collodio-albumen may also be employed. In either 

 case, the collodion used for making the plates must be absolutely struc- 

 tureless, for if it is not the magnified images will have a disagreeable 

 reticulated appearance. Pyrogallic acid is preferable to iron sulphate 

 for development, since it gives a much finer deposit. The process con- 

 sists in making a positive by copying an illuminated negative, a 1-in. 

 microscopical objective being used for this purpose. An apparatus 

 devised by Mr. Hislop, and described in Mr. Sutton's " Dictionary of 

 Photography," may be employed. It consists of a rigid mahogany board 

 about 6 in. wide and 3 ft. G in. in length. At one end two uprights 

 are fixed, between which a miniature camera, fitted with the micro- 

 scopical objective, can be moved up and down, so as to allow it to be 

 placed opposite the centre of the negative to be copied. The objective 

 is screwed to a brass tube, projecting from the camera towards the 

 negative, the tube being fitted with stops of various sizes. A micro- 

 meter head for the fine adjustment of the lens is also necessary, because 

 the majority of microscopic objectives are corrected only for the visual 

 rays. The sharpest visual focus must be found by means of a powerful 

 magnifying-glass, and the chemical focus ascertained by racking the 

 lens in or out to various distances until the proper chemical focus is 

 found. "When this has been done, the same correction may always be 

 applied unless the negative's distance from the lens is altered. The 

 negative is placed in a frame at the required distance on the long 

 mahogany board. The illumination may be natural or artificial, but 

 must, of course, pass through the negative. The variations of light, 

 negative, and collodion plate render it impossible to give any idea of 

 exposure. After exposure the little plate is placed under a low-power 

 Microscope, in yellow light, and a few drops of developer poured over 

 it. Development must be watched through the instrument, remember- 

 ing that a transparency is required, and that, therefore, rather greater 

 density than otherwise should be obtained. After fixing and drying, 

 the tiny plates are examined through a magnifier of about the power 

 which it is intended to subsequently attach to them, in order to see if 

 they are perfect and worth the subsequent trouble of mounting. The 

 photographs chosen are then cut into small squares with an ordinary 

 diamond. Care must be exercised that no dust adheres to the film side 

 of these small squares. The little lenses (or Stanhopes) to which the 

 view is to be cemented are now placed on the top of a small stove, and 

 very cautiously heated. A drop of Canada balsam is placed on the 

 end and allowed to soften, and the little square transparency taken up 

 in a pair of forceps and pressed — gently at first, afterwards more 

 strongly — into contact with the melted cement. The two are then 

 allowed to harden together for some hours. In order to be certain that 

 the operation has succeeded, and that the contact is perfect, the trans- 

 parency is examined through the rounded end of the little glass 

 cylinder, to which it is cemented, which acts as a Microscope, and gives 

 a magnified and distinct image of the object. If air-bubbles show they 

 are most likely due to unequal pressure in cementing the glass. The 



