1892.] MICROSCOPICAL JOURNAL. 13 



In both the " Zeiss " and Nachet prisms, which, by the way, are both 

 ci-edited to Nachet by Professor Zeiss himself, there is an awkwardness 

 of manipulation due to the necessity of placing the paper close to the 

 microscope stand, and especially if the stand in use be one of the larger 

 of the Continental stands or one of the English models. The Zeiss 

 prism is of such construction that it may be adjusted to throw the image 

 at an angle which, from its position, produced a distortion. The paper 

 must be propped up to prevent this. This defect was overcome by 

 Oberhiiuser, but his apparatus is inclined to strain the eye when con- 

 stantly used. The same is true of Abbe's camera lucida, but it is con- 

 stantly being improved. The first model of Abbe's had an arm to sup- 

 port the mirror shorter than those now recommended, and with the 

 larger stands consequently failed in their purpose. Those of his latest 

 pattern are so constructed that the mirror may be moved back and forth 

 on its supporting arm, which adapts them to the largest stands, but 

 adding a new, though less pronounced distortion. This is due to the 

 prism being the axis of the refracted image. 



The unequal reflections of light from microscope mirror and drawing 

 paper are partially equalized in Abbe's cameras by placing pieces of 

 glass of various density between the camera prism and mirror. In 

 those of Leitz's manufticture he has neatly arranged the glasses in frames 

 hinged in place to the camera, so that the\- may not be lost. 



Another improvement in Abbe's camera is the new method of turn- 

 ing back the camera proper from the centering collar when focussing 

 the object or changing the ocular without the necessity of unscrewing 

 the whole apparatus. Leitz has devised a substitute for this by simply 

 swinging the prism with its mounting to one side. 



Zeiss has promised a new sliding method of detaching the Abbe 

 camera at an early date, which will be awaited with interest by those 

 interested in the many valuable microscopic improvements made in his 

 factorv. 



RECREATIVE MICROSCOPY. 



Silver Crystals. — Ebbage, inthe English Ji^c//a;^/c, says, with an 

 emphatic caution, not to spill the solution because it stains black ; that 

 a pretty object is the arborescent growth of silver crystals. Dissolve a 

 small crystal of silver nitrate or a piece of lunar caustic in a few drops 

 of rain-water. Place a drop of this solution in the centre of a slip of 

 glass, and arrange it under a low power, concentrating the light fiom 

 above by means of a condensing lens. Now take a piece of copper wire 

 i^ inches long and bend it like the letter L, and bend the longer limb 

 in form of a hook, which will rest anchor-fashion when laid down. 

 Place this at the side of the drop of solution, allowing the hook to dip 

 into it at the edge. Chemical action results, copper going into solution 

 and silver crystalizing out while the process is watched through the 

 microscope. 



For Mounts. — Get your druggist to save you some of the powders 

 left after exhaustion by percolation. Many of them make fine micro- 

 scopic objects. 



