120 SUMMAKY OF CUERENT RESEARCHES RELATING TO 



recommend it with perfect confidence as one of tlie most efficacious 

 forms of drawing apparatus." 



It consists of two rectangular prisms, of which the smaller a is 

 placed over the eye-piece, while the larger h receives the reflection 

 of the drawing pencil. As the prism over the eye-piece is very 

 small, the observer can look past it at the image of the object in the 

 Microscope, whilst the paper and the point of the pencil appear 

 projected above it. 



Grunow's Camera Lucida.* —This is a modification of the Abbe 

 instrument, the mirror being replaced by a rectangular prism as in 

 the Nobert and other forms. 



Objectives of Large Aperture.!— Dr. J. Pelletan criticizes Dr. 

 Carpenter's remarks on this subject at Montreal,:}: and in regard to 

 injury to the eyesight suggested as the result of the use of a 4-lOths 

 in. of large aperture, considers it an " accusation as well founded as 

 to say that too large a hat will produce corns on the feet." If it is 

 the fact that certain objectives may injure the sight it is the high 

 powers with small angle, deficient in light, resohnng badly, and 

 requiring " efforts of vision," so to say, rather than the relatively low 

 powers of large angle, bright, and showing at once the image of the 

 object clearly resolved. 



He also claims as conclusive evidence in favour of the necessity of 

 objectives of the largest aperture the admission of Dr. Carpenter that 

 the flagella of Bacterium termo could not have been discovered with- 

 out such objectives. The ^-in. of 40^ could not be of any use to a 

 microscopist for delicate and serious observations, neither to the 

 diatomist nor to the investigator of the histological elements who 

 requires a large apertm'e to enable him to follow a fibril layer by 

 layer so as to determine all the relations of its position and the precise 

 point where it ends. 



Abbe's Method of Testing Objectives. — The late Dr. H. E. 

 Fripp published, in 1877,§ an account of Prof. Abbe's method of 

 testing the optical quality of objectives, which he suggested might be 

 usefully transferred to the pages of this Journal. Various causes 

 have hitherto prevented this, but we are now able to print it : — 



In ordinary practice, microscope objectives, if tested at all by 

 their possessors, are simply subjected to a comparison of performance 

 with other lenses tried upon the same " test objects." The relative 

 excellence of the image seen through each lens may, however, depend 

 in a great part upon fortunate illumination, and not a little upon the 

 experience and manipulative skill of the observer ; besides which any 

 trustworthy estimate of the performance of the lens under examina- 

 tion involves the consideration of a suitable test object, as well as the 

 magnifying power and aperture of the objective. The structure of 

 the test object should be well known, and the value of its " markings," 



* Amer. Mon. Micr. Journ., iii. (1882) p. 201 (1 fig). 



t Journ. de Microgr., vi. (1882) pp. 543-4. 



X See this Journal, ii. (1882) pp. 698 and 8.54. 



§ Proc. Bristol Natuialiats' Soc, ii. (1876-9) pp. 3-11 (2 figs.). 



