194 Corrections in the President's Address. 



On the evening of the 7th of March I also saw with this lens, 

 clearly and distinctly, the longitudinal lines of Nav. crassinervis, 

 for the first time in my life. Did the same again on the 13th. 



It will be inferred from the prominent place I. have given to 

 Stauroneis spicula throughout this trial, that I attach very great 

 value to it as a test for high powers. And such, indeed, is the 

 case. Of its extreme flatness, which recommends it for use with 

 objectives of the very finest and most delicate construction, I need 

 say little. There are other specialities connected with it. While 

 a good r^th may be able to show the striae pretty fairly, a still 

 better ^Vth will reveal still more ; and the best glass now in 

 existence will probably leave something, either in matter or degree, 

 to be shown by the still better objectives which may be used by 

 our aftercomers. Or to put it thus : if a person possess half-a-dozen 

 iVths, such that the second surpass the first, and the third the 

 second, and so on, each surpassing its predecessor by a specific 

 degree of excellence, and he try them upon it in the ascending 

 order, beginning with the lowest, he will hardly, when he has tried 

 his last and best glass upon it, rise up with the persuasion that he 

 has completely ecchausted his test. To be sure, the same might be 

 said, in a certain sense, of almost any test, but of none so truly, so 

 emphatically as of the one here mentioned. 



But after all it really does not matter very much what the 

 particular test is, if the operator only observe these three con- 

 ditions: (1) that the test employed be one that ought to be just 

 within the capacity of the class of objective he is trying ; (2) that 

 the operator be sufficiently exacting as to what constitutes the 

 best possible image ; (3) that he recollect with sufficient keenness 

 what that best possible image is. 



Corrections in the President's Address. 



By some unaccountable oversight, in copying out the data for 

 calculating the number of molecules in liquid water, the factor 

 expressing the specific gravity of the vapour of water was omitted, 

 and afterwards overlooked. The number of atoms of a gas should 



really be multiplied by f x 770 x Ta^oh ^ 3 = 617. But, 



moreover, on reflecting on the relative reliability of the determi- 

 nations by the various authors of the number of the atoms in gases, 

 it appears that in taking the mean, greater weight ought to be 

 allowed to that by Clerk-Maxwell, since founded on more recent 

 and accurate data. If his results be considered to be equal in value 

 to those of Stoney and Thomson combined, the mean would be 



