66 Bemarls on Mr. Carrutliers Views of Proiotaxites. 



I announced ttie intermediate beading of diatoms in May, 1869, 

 to ■which I here more particularly wish to draw attention, as they 

 exist in different planes of focal vision. I wnsh to add, that the 

 minute beading of the mid-rib described by me, did not appear more 

 than about one hundred thousandth of an inch in diameter under 

 a power of 1000 ; one of Browning's micrometer divisions then 

 corresponds to one milhonth upon the stage.* 



IV. — Bemarks on Mr. Carrutliers^ Views of Proiotaxites. 



By J. W. Dawson, LL.D., F.KS. 



In the 'Monthly Microscopical Journal' for October, 1872, Mr. 

 Carruthers, of the British Museum, has published a paper in which 

 he endeavours to show that mj Proiotaxites Logani from the Devo- 

 nian of Gaspe is a gigantic sea-weed, for which he proposes the generic 

 name Nemaiophycus. Though I saw this article some time ago, 

 other avocations have prevented me from attending to it until 

 now. 



The tone and manner of the article, I may say in passing, are 

 unnecessarily offensive ; and the author bolsters up his argument 

 by unfair assumptions that I am ignorant of some of the most 

 famihar facts of structinral botany, facts which were well known to 

 me while he was yet a school-boy, and which are stated or implied 

 in many of my papers on fossil plants. Possibly, however, Mr. 

 Carruthers is already aware of his bad taste in this matter, and it 

 will be to me a sufficiently ungracious task to expose, as I must do 

 in the interest of truth, the worthlessness of the explanation which 

 he offers of the nature of Prototaxites. I shall reply to his objec- 

 tions under the following heads: — (1.) The mode of occurrence of 

 Prototaxites. (2.) Its microscopic structure. (3.) Its probable 

 affinities. 



* With regard to these minute quantities, and to remove doubts which may 

 arise in some persons' minds as to the possibility of seeing such veiy minute linear 

 quantities, I may say that a minute of arc corresponds to the breadth of the 

 344th part of an inch as seen at ten inches, which is at least fom* times as thick 

 as a human hair at that distance. Now the one hundred thousandth of an inch 

 under a power of 1000 is precisely the same thing as a thousandth of an inch 

 under a power of one, or seen naturally at ten inches. But we can see hairs much 

 finer than this — say three times — therefore, with regard to arc, we can see with a 

 power of 1000 the \ of ^ooaoo *-^- '^^^^ ^ power of 3000 about the millionth. 

 To find the angle in seconds 1" = 0-000004848 = -—g-^^^th, nearly. 

 The angle under a power of 3000, j 3000 3 



at a distance of ten inches, is for > = -— z-r^rr-- =: r--r7-: 



a millionth of an inch .. ..( 10x1000000 10000 

 Divide this by the value of one second and we get six seconds in the angle 

 subtended by mooooo ^^ under a power of 3000. 



