135 



NOTES AND CORRESPONDENCE. 



Angular Aperture, — My object in the paper on the subject of 

 angular aperture, which you were good enough to insert 

 (p. 256, last volume), was simply to facilitate the application of 

 Mr. Lister's method of measurement, by showing how that 

 method might be made available independently of the special 

 apparatus usually considered requisite for this purpose. Mr. 

 Hendry, therefore (p. 61, present volume), is mistaken if he 

 supposes, as he seems to do, that I claim for the method, as 

 described by me, superiority in point of accuracy to the 

 method as usually practised. I do not do that, but I claim 

 for it equality in this respect. An angle is determined quite 

 as accurately by measurement of the sides of the triangle to 

 which it belongs, as by measurement of its subtending arc. 

 The use of two candles saves the trouble of moving the one 

 candle, if one only is used, from one side of the field of view 

 to the other; and the indication of these being properly 

 placed will easily be found to be in exact accordance with 

 the corresponding indication in Mr. Lister's method, as 

 usually described. 



But I am surprised at Mr. Hendry's statement, that " my 

 rule gave no provision for angles exceeding 90°." I know of 

 no ground for this statement. Take his fourth example : 



Lights apart, 44 inches ; distance of lens, 10 inches. 



Hence 41 ■*■ (10 x 2) = 22. On reference to Hutton's 

 Tables, I find this to be the tangent of 65° 33'. The aperture 

 therefore is 131° 6'. 



Mr. Hendry, perhaps, has only a table of logarithmic 

 tangents. Very well. The logarithm of 2"2 is 0*3424, to 

 which adding 10, to accommodate it to the tabular radius, it 

 becomes 10 , 3424 ; and this is the logarithmic tangent of 

 65° 33', as before.— M. Gray, 7, St. Paul's Villas, Camden 

 Town, December 5th, 1859. 



A New Cement for mounting Objects for the Microscope, either 

 in dry cells or in fluid. — I have found that great rapidity is 

 obtained in mounting objects in a cement made with as- 



