T. F. SMITH ON ARACHNOIDISCUS. 249 



2 glass was at its best at point 5 ; No. 3 at point 1\ ; and the 

 last glass at its best on the same slide at point 10, or as far as it 

 could go. It is no use blaming opticians, for the English micro- 

 scopists have been brought up (and rightly, up to a certain point) to 

 believe in the Podura scale, and makers cannot be expected to run 

 the risk of producing a glass that is not at its best on that test. 

 The only way, then, is to offer a substitute that shall stand for the 

 oil-immersion in the same relation as the Podura scale does to the 

 dry glass, and for that purpose I beg to offer the outer plate of the 

 Arachnoidiscus (anything) mounted in balsam. 



To me there is a particular appropriateness in choosing this as a 

 test object, from the fact that, although its main features for the 

 last forty years have been as well known as the Podura scale itself, 

 the discovery of the finer markings or structure is due entirely to 

 the oil-immersion objective. 



The first public notice and figuring of this diatom I can find 

 occurs in a paper read by Mr. Shadbolt before the Microscopical 

 Society on November 14th, 1849, and is found in Vol. III. of the 

 Transactions of that Society. The plate accompanying that paper 

 is copied into the plates in Pritchard's " Infusoria," and more 

 recently a fine plate of the same diatom is found in Dr. Carpenter's 

 "Microscope."* In Schmidt's "Atlas," also, are numerous drawings 

 of this diatom, and recently Mr. Morlaud has described its structure 

 to you, so that it is no stranger 1 am bringing before you to-night. 

 In all the drawings of this diatom, the markings on the side from 

 which it takes its name are shown as oval areolation only, and this 

 may be taken as all that has been seen up to the last two years. 

 But in the plate accompanying the paper by Messrs. Nelson and 

 Karop, taken as read on January 28th, 1887, the new structure is 

 shown at Fig. 4. 



The same appearance was familiar to me at the time, but I am 

 certainly indebted to those gentlemen for a true knowledge of its 

 character, as before that I had taken it as due to perforations in 

 the outer membrane. I am now convinced, however, that the 

 structure is as they say, " points projecting into the areolation 

 from its edge." To me the advantages of this new test-object for 

 an oil-immersion are, that the little projecting points, or spines, 

 can only be clearly defined where the objective is perfectly correct, 



* Eeproduced from a drawing by the late Richard Beck, and published 

 in his treatise on the use of the microscope. — [Ed. " J. Q. M. C."] 



