276 SUMMAEY OF CaRRENT RESEARCHES RELATING TO 



on the top of tlie stem. The second plate is moved by two knobs at the 

 sides. 



A blackened brass plate slides beneath the glass stage, so that it can 

 be used for both transparent and opaque objects. 



Two large wooden hand-rests, similar to those of Mayer's dissecting 

 Microscope, fit on pins (not shown in the fig.) at the sides of the stage. 



Henri CI, J. F., and 0. C. Mellob. — An Old Microscope of the Culpeper Type. 

 [Same model as figured on Plate IV. of ' Adams' Essays on the Microscope,' 

 1787.] I'roc. Ainer. Soc. Micr., X. (1888) pp. 140-2) (1 fig.). 



PiEKsoL, G. A. — Continental Microscopes. 



Queen's Micr. Bulletin, V. (1888) pp. 23-4. 



(2) Eye-pieces and Objectives. 



Beck, C. — The Construction of Photographic Lenses. 



[" The achromatic Microscope was worked out by Lister and others by i^ractical 

 methods, and even at the present time many things are done in practice which 

 are not even known of by theoretical men. I believe I am correct in saying 

 that there is no book which gives a correct representation of a high-power 

 microscopic object-glass, and most of the figures which are to be seen in books 

 are entirely misleading." Also remarks on Jena glass.] 



Journ. Soc. of Arts, XXXVII. (1889) pp. 180-92 (6 figs.). 

 Detmers, H. J. — American and European Microscopes. 

 [Controversy as to Objectives.] 



Froc. Amer. Soc. Micr., X. (1888) pp. 149-54 ; of. also The Microscope, IX. 

 (1889) pp. 14-15, and St. Louis Med. and Surg. Journ., LV. (1888) p. 348 ; 

 also Dr. J. Pelletan in Journ. de Microgr., XIII. (1889) pp. 101-4. 

 EwELL, M. D. — American Objectives and Dr. Zeiss's Apochromatic Objectives. 

 [Opinion unfavourable to the latter.] 



The Microscope, IX. (1889) pp. 30-1. 



Heukck, H. van. — Les Apochromatiques juges en Amerique. (The Apochro- 

 matics judged in America.) Journ. de Microgr., XII. (1888) pp. 438-40. 



James, F. L. — The Old Nonsense still on its Rounds. 



[Comments on the " Wonderful Swedish Optical Glass " paragraphs. See this 

 Journal, 1888, p. 499.] 



St. Louis Med. and S,irg. Journ., LV. (1888) pp. 350-1. 



(3) Illuminating- and other Apparatus. 



Ahrens' Modification of Delezenne's Polarizer.* — Mr, C. D. Ahrens 

 has devised a modification of Delezenne's polarizer, which consists of a 

 total-reflection prism combined with glass plates and black glass mirror, 

 arranged so that the polarized beam is parallel to the original one. 

 The combination of plates and mirror is adopted so as to give enough 

 light and still keep the polarization sufficiently good. One or two 

 plates laid over the mirror are found to give the best results. The fact 

 that a beam polarized by reflection is not coincident with the original 

 beam renders it inconvenient if not impossible to rotate the polarizer, 

 and to overcome this defect Dr. S. P. Thompson has arranged two 

 quarter-wave plates, one of which may be rotated. The first plate 

 circularly polarizes the plane-polarized beam, and the second (or 

 rotating one) re-plane-polarizes it in any desired plane. 



Falter's Rotating Object-holder. — This apparatus (fig. 42) of 

 Messrs. G. Falter & Son of Munich is intended to provide a rotating 

 object-holder which can be adapted to any Microscope. 



The objects are arranged round the circumference of a glass disc 



* Nature, xxxix. (1889) p. 358. 



