120 



THE AMERICAN MONTHLY. 



[June. 



spent nearly a year trying to find a mi- 

 crometer without sensible error, and if the 

 Doctor has one that is absolutely without 

 error he is indeed fortunate. I have one 

 screw stage-micrometer in which I have 

 never been able to discover any error 

 whatever. It was made by Geo. Clark, 

 of Alvin, Clark & Co., and was formerly 

 owned by Prof. Rogers, who states that 

 he was unable to find any error in it. It 

 is the only precision screw I have ever 

 seen that is without error. The accumu- 

 lated errors of a single revolution of the 

 screw of my dividing engine amount 

 to yyijy^ of an inch, and 1 do not suppose 

 that the screw of my film micrometer is per- 

 fect, though it is a very good one. Will the 

 Doctor kindly give us his experience with 

 his 3^ -inch periscopic eye-piece. I use a 

 Huygheinan i-inch, and if I were to order 

 another would have a i>^-inch. In or- 

 dinary comparisons, I use a Bausch & 

 Lomb X-inch opaque illuminator with a 

 prism above the front lens, which gives 

 excellent results on metal surfaces. My 

 experience is that there is no micrometer 

 at all comparable with one solid on metal. 

 I use principally one sold by Prof. Rogers 

 on speculum metal, which I have for 

 some months lieen comparing with the 

 standard of the American Society, so that 

 its errors, which are very small, are well 

 known. Its correction for total length 

 (icm.) is only -I-0.25 //. 



The subject of micrometry is an inter- 

 esting and important one, and is not en- 

 tirely free from difficulty ; and a free in- 

 terchange of opinion and judicious criti- 

 cism of methods and results cannot fail 

 to result in good. 



Chicago, III. M. D. Ewell. 



MICROSCOPICAL SOCIETIES. 



Washington, D. C. 



Forty-fourth regular meeting. May i ith. 



Mr. Frank H. Knowlton presented a 

 communication entitled ' A Resume of the 

 Algo-Lichen Hypothesis,' printed in full 

 on another page. In the discussion 

 which followed Prof. Seaman said that 

 Schwendener's theory had been the sub- 

 ject of many attacks. It seemed repug- 

 nant to the laws governing plants so 

 highly organized as the lichens. Their 

 growth is extremely slow and, to a certain 

 extent, peripheral, therefore the centre 

 might die and the surface grow, but this 

 did not appear to be the case. One of 

 the principal objections to the theory is 



the variation in habits of plants which it 

 requires to make it tenable. This varia- 

 tion should come, if at all, from the fun- 

 gal side, but this does not appear to be 

 the fact. See paper by Metcalf Johnson,* 

 accompanied by a plate showing spores. 

 The theory offers a tempting field for 

 investigation. He had never seen gonidia 

 except fully developed. 



Prof. Seaman showed specimens of an 

 alga from a pond in the monument lot. 

 The algae formed a layer on the water 

 I mm. in thickness. He had not quite 

 made out their nature. Prof. Burgess 

 thought they were Microcystis. Dr. Rey- 

 burn stated that he had been investigating 

 Laverau's researches on the presence of 

 an amoeboid body in blood corpuscles in 

 cases of acute intermittent fever and prom- 

 ised to give an account of his results. 

 Mr. Knowlton suggested that all botanical 

 observations be recorded in the form of 

 a card-catalogue. 



E. A. Balloch, Rec. Seer. 



NOTICES OF BOOKS. " 



What is Medicine? Annual address de- 

 livered before the American Academy 

 of Medicine, at New York, October 

 28, 1886, by Albert L. Gihon, A. M., M. 

 D., Medical Director, U. S. Navy, 

 President of the Academy. Philadel- 

 phia, 1886. (Pamphlet, pp. 28.) 



We are always glad to see the outspoken 

 and telling writings of Dr. Gihon, whose 

 eftorts to improve and elevate the stand- 

 ard of education in medicine are praise- 

 worthy and, we hope, of great influence. 

 The profession at large is responsible for 

 the condition of the ordinary medical 

 colleges — how long is this disgraceful 

 condition to continue ? Those who do 

 not know the extent of the evil should 

 read Dr. Gihon's writings. 



Exchanges. 



[Exchanges are inserted in this column without 

 charge. They will be strictly limited to mounted ob- 

 jects, and material for mounting.] 



For Exchange : Very rich specimens of Isthmia 

 nervosa in situ on seaweed, from Timber Gulch, 

 Cal., for good slides of diatoms or diatomaceous ma- 

 terial or Foraminifera. 



L. M. KING, 

 Santa Rosa, Cal. 



* Monthly Micr. yoitrn. vi., 217. ' The Monad s 

 Place in Nature.' 



