MEMORANDA. 157 



of M N. Its length may be calculated with the greatest ease 

 by the help of the ordinary logarithmic tables of sines. In 

 comparing chords, we use the sines of half their including 

 arcs, as in the example which is appended : — 



Example. — See Fig. III. 



The arc A a is found = 38° 

 Required the length of M N. 



Proportion. 



Sine 4° : -0100244 Millim. : : Sine 19° : M N. 



Calculation. 



Log. -0100244 Millim. == 2-0010592 

 Log. sine 19° = 9*5126419 



7-5137011 

 Subtract Log. sine 4° = 8-8435845 



MN. = Num. -0467861 Millim. = 2-6701166 



In like manner the length of any other chords of the dotted 

 circle may be easily determined ; and a table — if required — 

 drawn up, from which the measure corresponding to each 

 degree of the scale can, by mere inspection, be at once ascer- 

 tained. 



When the eyepiece rotates smoothly in the tube of the 

 microscope, and a magnifying power of 500 or 600 diameters 

 is used, measurements may be made with such an instrument 

 with the utmost nicety. Welcker recommends that the top 

 of the tube of the microscope should terminate in a hollow 

 cone, into which is received a conical collar, supporting the 

 pointer, and slipt on immediately beneath the milled rim of 

 the eyepiece. The errors of manipulation should hardly 

 exceed l-40,000th of an inch, — a degree of exactitude scarcely 

 attainable by the cobweb screw-micrometer. An instrument 

 constructed for me, by Mr. James Bryson, of Edinburgh, on 

 the plan above described, has been tried against a finely- 

 finished screw-micrometer, and found to perform with very 

 great accuracy. — W. Robertson, M.D., Monthly Journal of 

 Medical Science. 



Cilia in UniceUuiar pianta. — In consequence of Several com- 

 munications appearing in the ' Microscopic Journal,' announc- 

 ing the discovery of the existence of cilia both externally and 

 internally, in the DesmidiecB and Diatomacece, I have been 

 induced to make a careful series of examinations of some 

 of these objects, under all varieties of illumination, dif- 

 ferences of aperture, and magnifying power. 



