Royal Microscojpical Society. 255 



spection or otherwise air-bubWes are apt to form in the tissue. It is 

 best to use a long-necked bottle and to hold the mouth close to the 

 negative so that none of it shall drop out but shall flow gradually. 

 If a tendency to air-bubbles is shown, the solution had better be 

 thinned with ether ; a very small quantity of Canada balsam added 

 to it dmiinishes the tendency to this defect, but causes reticulations 

 in the film, which, however, do not show in the print. At first the 

 film becomes of an opaque white colour, but it clears after a time and 

 becomes quite transparent ; the plate is then put into water and after 

 a few hours the tissue will come off easily, bringing the negative with 

 it. It is not necessary in this part of the process to rub the plate with 

 wax, previously to taking the negative. The convenience of this 

 tissue beyond the facility of carriage is, that if one has two or three 

 negatives of diiferent examples of the same kind of cells for instance, 

 they can be printed together by simply cutting them out of each 

 and sticking them on to a sheet of the tissue by means of gum, when 

 a print of the whole can be taken, and if the negatives are ef dif- 

 ferent densities, the weaker ones can be covered over for a part of 

 the time of exposure. It is often necessary to paint out the back- 

 ground of a negative for this purpose ; nothing is better than Bate's 

 black varnish ; this works very well with turpentine, and should be 

 applied under a simple microscope of considerable power, or by the 

 compound microscope with the erector, so as to get the edge tho- 

 roughly smooth. I hope in the preceding remarks that I have done 

 something towards simplifying the process under consideration, a 

 process which although I imagine not destined to supersede the 

 pencil, yet has such great advantage that it ought to be encou- 

 raged. Its advantage may be summed up in one word of great im- 

 portance to scientific men, who are or ought to be searchers after 

 truth, — that one word is accuracy ; whatever is in focus on the slide 

 will reappear in the negative. On the other hand its disadvantages 

 are twofold ; one which appears insuperable is that it only shows 

 objects in one plane, the other is that sunshine is necessary. This 

 in a climate like ours is very serious, but it may be overcome by 

 patience and waiting for a fine day ; nevertheless it is sufficiently 

 provoking to have one's work interrupted by a sudden overclouding 

 of the sky. Whoever will invent a steady light of great actinic 

 power, which shall be inexpensive, not requiring quarts of acid or a 

 small steam-engine, will confer a benefit on the science of anatomy. 



