138 



THE AMERICAN MONTHLY 



[July, 



one with refractive index of 1.7, the other 

 of 2.0. The popular Pillsbury cabinets 

 can also be obtained from them. 



— Messrs. H. R. Spencer & Co. have 

 issued a new price list of objectives, 

 which can be obtained by addressing 

 them at Geneva, N. Y. They make 

 now a variety of lenses, all of them 

 good, but their highest quality lenses 

 are to be especially commended. They 

 offer a ^^^-inch, b. a. 116°, n. a. 1.29, for 

 $60.00, and speak in high praise of it, for 

 its long working distance and resolving 

 power. Another -^ in the same series, 

 having a B. A. of 125°, N. A. 1.35, costs 

 $80.00. The higher angle objectives are 

 ' guaranteed to equal in performance any 

 that can be made.' 



— The Gundlach Optical Company has 

 been very busy of late filling orders and 

 preparing an extensive exhibit of photo- 

 graphic goods for the Photographic Con- 

 vention at St. Louis. We may say, in 

 passing, that they are doing some good 

 work in objectives for field photography, 

 a line of business they have recently 

 established, although Mr. Gundlach has 

 long been identified with the manufacture 

 of such lenses. We are pleased to see 

 evidences of their prosperty. 



— Dr. Piersol, of Philadelphia, has gone 

 to Germany, where he intends to pursue 

 his studies in histology. He will also 

 continue his work in photography, in 

 which he has been so successful. Our 

 readers are already indebted to him for 

 some valuable contributions to these col- 

 umns, and may expect others during 

 his sojourn in Leipzig, where we trust his 

 experiences will be always pleasant and 

 profitable. 



— Dr. Henri Van Heurck has had re- 

 markable success in photographing the 

 Amphipkiira and the i8th and 19th bands 

 of Nobert. We are in daily expectation 

 of receiving prints from some of his recent 

 negatives, which he has promised to send 

 us, but in anticipation of their arrival it is 

 of interest to have the opinion of such an 

 experienced microscopist as Dr. Royston 

 Pigott, who has thus expressed his ap- 

 preciation in a private letter. He writes : — 

 You will not be astonished when I declare 

 they have in my opinion no equals. The 

 total disappearance of false lights along 

 the margins, which people have almost 

 doted in declaring to be quite unavoid- 

 able, because they were diffractions, 

 utterly explodes that gratuitous presump- 

 tion. 



CORRESPONDENCE. 



An Old Record of Spencer. 

 To THE Editor : — I have somewhere 

 heard it said that in one of the editions 

 of Ouekett's Treatise on the use of the 

 Microscope there appeared an account 

 of the performance of the objectives of 

 the late Chas. A. Spencer, which were 

 then just beginning to be known in 

 Europe, and also an engraving of what 

 was then the test object, Harictila Hip- 

 pocampus, as shown by a Spencer ob- 

 jective. Also that the fact of such no- 

 tice aroused such a feeling in England 

 that the objectionable matter, together 

 with the cut, was omitted in subsequent 

 editions. I would like to know if such 

 was the fact, and, if s'o, in what edition 

 of Ouekett the notice appeared. 



W. 



Measuring Blood Corpuscles. 



To THE Editor : — I desire to present 

 the following reply to Dr. Ewell's commu- 

 nication, in the June number of the Journal, 

 on fine measurement. 



My stage micrometer has no appreciable 

 error when compared with the U. S. Coast 

 Survey standard. The micrometer screw 

 was made especially for micrometer use. 

 A leading firm of fine tool makers refused 

 to fill an order for a screw which they 

 would guarantee to be accurate, conse- 

 cjuently Mr. Fasoldt assumed the task. 

 After many trials and corrections, occupy- 

 ing several months of time, a screw of 

 1 00 threads per inch was produced, which 

 accorded in every revolution with the 

 standard stage plate. The error, if any, 

 of this screw is extremely small. 



The use of a micrometer requires care 

 and frequent testing, since the ordinary 

 wear, when in service, tends continually 

 to change the rate of the screw, and this 

 is especially the case if a certain j^^ inch 

 or gJjj inch of its length is used when 

 making a large number of blood meas- 

 urements. 



The measurement and comparison of 

 lines and screws requires much technical 

 experience, and this matter is quite inde- 

 pendent of mathematical ability. In 

 measuring with a screw the milled head 

 should be invariably turned in one direc- 

 tion, to the limit of the division or to the 

 end of the scale to be measured. If a 

 division or line be inadvertently overrun, 

 the screw should be reversed one or two 

 revolutions and the line again approached 

 carefully in the proper direction. The 



