ERRORS OF INTERPRETATION. 165 



ground edges, and bring one end in contact with the 

 drop of blood, as shown in fig. 97, at an angle of 

 45 ; then draw it evenly and quickly across the uiider- 

 slide, and the result will be to spread out the corpus- 

 cles evenly throughout. The blood discs being lenti- 

 cular bodies, with depressed centres, act like so many 

 little glass-lenses, and show diffraction rings if the 

 light is not properly adjusted. In the Journal of the 

 Royal Microscopical Society, page 542, vol. iv., Dr. Seiler 

 fully describes the arrangement of the lamp, condenser, 

 and mirror. 



Errors of Interpretation. To be in a position to 

 draw accurate conclusions of the nature and properties 

 of the object under examination is a matter of the 

 greatest importance to the microscopist. The viewing 

 of objects by 

 transmitted light 

 is quite of an ex- 

 ceptional charac- 

 ter, much calcu- 

 lated to mislead 

 the judgment as 

 well as the eye. 



It requires, there- FlG . ^.-Seller's 



lore, an unusual 

 amount of care to avoid falling into errors of inter- 

 pretation. There are perhaps no set of objects with 

 which I have become acquainted, and which have given 

 rise to more discussion as to the precise nature of their 

 structural elements than those of certain of the diato- 

 maceee. The minute scales of the Podura (Lepidocyr- 

 tus cervicollis') and their congeners Lepisma saccharin 

 are equally debatable. Mr. E. Beck, in an instructive 

 paper published in the Transactions of the Royal 

 Microscopical Society, says that the scales of the latter 

 can be made to put on an appearance which bears 

 little resemblance to their actual structure. 



On the more abundant kind of scales the prominent 

 markings appear as a series of double lines, these run 

 parallel and at considerable intervals from end to end 

 of the scale, whilst other lines, generally much fainter, 



