MICROSCOPY. 359 



off 1.1, we should have for balsam angle: sine \ aperture 



=0.733 = sine 47 15'; twice which, or 94 30' = balsam 

 1.5 



angle. If the objective had, instead of 1.1, given 1.25 as 

 numerical aperture, its balsam angle would have been 113. 

 We have, then, in these two cases of 1.1 and 1.25 a resolving 

 power in the first 10 per cent., and in the last 25 per cent., 

 greater than the possible limit of a dry lens (sine 90 = 1) 

 {Journal of the Royal Microscopical Society, March, 1878). 



New Form of Micrometer. 



Mr. G. I. Burch has described a micrometer based upon a 

 comparison of the reflection of a scale with the image of the 

 object, and which he claims is equal in accuracy to all other 

 micrometers except the Cobweb. It consists of a cap, fitting 

 over the eye-piece, containing a piece of neutral-tint glass (or 

 looking-glass, with the amalgam removed in the centre), set 

 diagonally, so as to reflect to the eye the image of a scale, 

 which is carried by an arm ten inches long, attached to the 

 cap, the object being observed in the usual way, through the 

 eye-piece, but looking through the diagonal glass. To ad- 

 just the scale so that it may read decimals of an inch, etc., 

 it is moved on the arm nearer to, or further from, the eye, 

 till, on adjusting the focus so that the apparent distance of 

 the two images may coincide, every tenth division on the 

 scale shall cover the -j-^j or the 1( / 00 of the stage microme- 

 ter, according to the power used {Journal of the Quekett 3Ii- 

 croscopical Club, No. 37, 1878). 



New Test -object. 



Professor Ranvier recommends as a test for objectives 

 intended for histological work ("which are required, not 

 for flat bodies presenting only fine striae, but for objects of 

 irregular and varying forms rough, concave, or convex") 

 the isolated muscular fibrillae of the wings of the Hydrophi- 

 11. With a power exceeding 300 diameters, the alternate- 

 ly thick and thin dark disks which characterize the fibrillaB 

 may be seen. Although Professor Ranvier, from whose book 

 on "Practical Histology," just published, the preceding is 

 taken, is the leading histologist of the day, yet he seems 

 to be quite ignorant of the principle involved in the con- 



