108 MEMORANDA. 



not o^og^jTj = 100 in the slide^ it must be accounted 

 " very rare ; " above 100, but not exceeding 500, " rare " 

 only, this giving almost one to every field of a "I" object- 

 glass. 



And this leads me to observe that when the specimens are 

 small, or so numerous that tliey are entitled to be called 

 " common," they ought to be seen in every field of a " j " 

 object-glass ; and then we may decide on the appellation to 

 be given by counting those observed in the field at one time, 

 or by taking an average of two or three fields. The object- 

 glass I employ has, with a low eye-piece, a field of "03 inch 

 in diameter, and therefore an area of about '0003 nearly ; 

 hence to be " not common," each species ought to occupy 

 from ■"^"^Oy'^ to •«/„o_3^ that is from -0000006 to -0000015 ; and 

 to be '" common" from -0000015 to -0000075. In the case 

 of Ca'hpylodiscus costatus already mentioned, its area is 

 •00001 ; and as the greatest of these numbers (-0000075), 

 divided by -00001, only gives -75 or |, only three specimens 

 may be expected in four fields of view ; and therefore, 

 although one only occurs in every field, the species ought to be 

 esteemed " very common." In Eunotia tetraodon, the sur- 

 face is -000003 ; hence, when there are from one to four in 

 the field at once, it may be called " common," more than 

 that " very common." Navicula cryptocephala has a surface 

 of -0000004; andas:ggg§§§f = f ; so, unless are there at least 

 three specimens to every two fields of view, the species must 

 be held as only "not rare," if, indeed, it be not "rare," 

 or " very rare," when we must have recourse to the area of 

 the cover. 



A very slight practice will render the actual calculation 

 unnecessary ; but there ought to be some check, for many slides 

 sold are almost worthless, from not containing specimens of 

 the species labelled, and for which it is purchased, in a suffi- 

 cient number to illustrate its variations. — A. 



The Microscope as a means of Diagnosis. — Singular case of 

 Intestinal Concretions. — The value of the microscope as a 

 means of diagnosis is now universally acknowledged by every 

 medical man. Many are the instances I could enumerate, 

 in which, without its assistance, no clear or definite opinion 

 could be arrived at. Among the many cases which have 

 come under my observation, the following one may not be 

 uninteresting, as I know of no other similar case, save one 

 .nentioned by oiu' esteemed friend, Mr. Quekett, in his first 

 volume on ' Histology.' I give you the history of the case 



