Royal Microseopieal Society. 251 



a treble fold of black calico long enough to reach from one end of 

 the board to the other ; this calico bag is kept apart by two rows of 

 rings which run along a couple of brass rods attached one to each 

 upper angle of the focussing frame ; the whole is so arranged that 

 the picture from the object under the microscope falls on the centre 

 of the focussing glass, which is made by pouring a very thin solution 

 of starch over a piece of patent plate, and allowing it to dry sponta- 

 neously in a horizontal position. When the apparatus is required 

 for use it is placed on a table, the microscope is fixed in its proper 

 position ; the body being arranged horizontally is pushed through 

 the opening for the lens in the front frame, and is surrounded by 

 black velvet, so as to make the aperture impervious to light. The 

 focussing frame is fixed at any point on the board according to the 

 magnifying power required ; the fine adjustment is moved by means 

 of a rod attached to the side of the board, the further extremity of 

 which carries a small grooved wheel which moves the fine adjust- 

 ment by means of an elastic band. The only other piece of appa- 

 ratus required is a small glass cell filled with a solution of alum, 

 which cuts off the heat rays of the sun without in the least dimi- 

 nishing the fight. The eye-piece is always taken away from the 

 microscope, as its presence diminishes the fight and the definition, 

 the increase in size of the image being obtained by a method of 

 enlargement to be mentioned presently. With the above arrange- 

 ment the ^th object-glass gives a magnifying power of 350 

 diameters. The advantage of employing the microscope itself 

 instead of having the object-glass fixed to a special frame, as some 

 recommend, is obvious, for if anything occurs in one's researches a 

 copy of which it would be advantageous to keep, it can be photo- 

 graphed at once (provided the sun shines), with less difiiculty than 

 by using the camera lucida. There is a good deal of trouble atten- 

 dant on getting the focus properly ; with the 4th or ifh and higher 

 powers the image looks scarcely more defined on the focussing glass 

 when it is in focus than when it is just out of it ; a magnifying glass 

 must be used, a watchmaker's lens, or an ordinary doublet does 

 very well. When the object-glass is just within focus there is to be 

 observed round the external edge of the subject to be photographed 

 a border of white light ; as the object-glass is being moved away, 

 this border diminishes in width, and just as it gets out of focus 

 the bright border suddenly changes to a dark one ; the moment 

 must be seized when this bright border is on the point of dis- 

 appearing and before the dark edge is seen ; at this point the object 

 is exactly in focus. To get the best eSect the adjustment for 

 covered objects must be screwed down, and the thinnest possible 

 covering glass (0-005 inch and less in thickness) must be em- 

 ployed. If this is not done, concentric fines, called interference 

 lines, are apt to surround the subject, spoiling the efiect and 



