NOTES AND MEMORANDA. 



65 



After the image, complete in all its details, has by this means 

 been developed, it is washed by pouring water over it, and fixed by 

 dipping it in a solution of hyposulphite of soda (1 to 10). It is 

 again washed, dried, and varnished. 



The process of taking prints is conducted in the same way as in 

 ordinary photography ; a very practical and simple method is that by 

 means of sulphate of iron. A durable prepared paper may be 

 obtained from Marion and Gevy, of Paris, which gives excellent copies 

 in blue colour, without any special skill being required. 



Measure for Covering Glass. — For the exact measurement of the 

 thickness of covering glass to hundredths of a millimetre two dif- 

 ferent mechanical appliances have hitherto been employed — the screw 

 and the lever. The editor of the ' Zeitschrift fiir Mikroskopie ' * 

 points out that the same object may be attained by a suitable adapta- 

 tion of a movable wedge, the measuring wedge invented by P. Schone- 

 mann, which is distinguished by its great simplicity and solidity, and 

 has recently been considerably improved. 



The geometrical principle of the ajDparatus is as follows : — If a 

 right-angled triangle ab c (Fig. 1), whose hypothenuse ac = 5 cm.. 



Fig. 1. 



and its perpendicular ah = 1 cm., moves between the fixed lines m n 

 and op in such a way that ac slides along wn; then the line ch 

 (1) always remains parallel to op, (2) the distance between the movable 

 line cb and the fixed line op will always be one-fifth part of the dis- 

 tance which the point c, or any other point of the hypothenuse a c, has 

 moved from its original position. 



If, for instance, the triangle ab c moves to a a' b', the point c will 

 have moved over the distance a c = 5 cm., while the line c b will have 

 moved to the distance a 6 = 1 cm. from the fixed line op. 



It is on this principle that the construction of Schonemann's gauge 

 is based, as shown in Figs. 2 and 3. 



On one of the long sides of a brass base-plate a scale is fixed, 

 the divisions on which are half a millimetre apart. On the other 

 long side is a piece of brass bent inwards {d. Fig. 2). Between 

 these a wedge, provided with a nonius, can be moved backwards and 



Vol. i. p. 283. 



VOL. II. 



