REVIEW. 173 



disclaims (and we think with good reason) an unreasonable 

 contempt for these minutise. Doubtless a great deal of time 

 has been wasted in ''resolving diatoms," and if that were 

 an end in itself, it would be a ver}' poor one ; but there can 

 be no question that these minute observations liaA'e done 

 much to stimulate the energy and skill of microscope makers. 

 Moreover, microscopes are actually corrected — that is, vir- 

 tually made — by the help of these test objects. Herein lies 

 the fallacy pointed out by Dr. Royston Pigott ; the instru- 

 ment is made to show a certain appearance (such as exclama- 

 tion marks on the Podura scale), and altered if it does not 

 show it. Of course, then, it will always exhibit the same 

 structure, but the value as an independent test is gone, since 

 we do not know that the appearance may not depend on 

 some fault in the glass which we go on perpetuating. This 

 source of error is not noticed by Professor Frey, but he gives 

 one additional test, too little used by our makers, namely, 

 Nobert's lines. Even these, however, test one quality only of 

 a microscope. The only really independent test is that sug- 

 gested by Dr. Pigott, the image formed by the microscope of 

 a known object visible with the naked eye. Here we have at 

 once a standard of comparison, and the photographic achieve- 

 ments of our American colleague, Dr. Woodward, remove 

 another element of much uncertainty, the interpretation of 

 observations with a view to their representation. 



These data supply the bases for a really scientific system of 

 testing microscopes, which has yet to be constructed. 



Perhaps the most valuable portion of Professor Erey^s work 

 is that relating to the methods of preparing histological 

 objects, and the various sections on staining, injecting, and 

 mounting. Por the purposes of the medical student and 

 histologist there is no doubt that this is by far the most 

 useful and complete body of instruction that has ever been 

 put together ; it stands, indeed, without a rival. The happy 

 eclecticism of the writer preserves him from the one-sidedness 

 which is the great defect of some of our English text-books ; 

 and though his own technical skill is well known, he has 

 not been unwilling to receive hints from all quarters. 



It is, of course, impossible to notice the multifarious topics 

 of this part in detail ; but we may note in passing that Dr. 

 Frey still looks upon carmine as the best colouring material, 

 and, for permanent preservation of objects, upholds Canada 

 balsam in preference to dammar or other substitutes lately 

 recommended. 



He also shares the distrust of many skilful histologists for 

 mechanical appliances intended to come in aid of manual 



XI. — NEW SEK. M 



