124 



THE AMERICAN MONTHLY 



[July. 



lion in saying that similar abnormal- 

 ities are constantly recorded as truths, 

 and for that same reason which causes 

 excellent observers to differ in a most 

 remarkable manner, about the appear- 

 ances of the same object under dififer- 

 ent microscopes. It has been put 

 very forcibly by Dr. Dippel, that if the 

 shape of the object is unknown, cor- 

 rection may be a mistake, and that 

 when the focus is at a lower plane 

 than the summit of the object, correc- 

 tion may positively mislead. 



There is no doubt that an image 

 seen under a certain correction, and 

 which is stated to be normal, is mod- 

 ified by under- and over- correction. 

 How to get at the truth is difficult, 

 except in the instance of geometrical 

 bodies and definitely parted lines, but 

 the examination of the same object 

 by many observers with different in- 

 struments gives experience ; and with- 

 out indulging in calculations, including 

 the method of least squares, it is final- 

 ly settled that such and such is the 

 real shape. 



The possibility of error remains, 

 however, and it is perfectly evident 

 that many a difference of delineation 

 of carefully investigated objects re- 

 sults from non-correction and over- 

 and under-correction. One cannot 

 but help thinking that the difficulties 

 in correcting dry objectives of high 

 amplifying power and great numerical 

 aperture, will lead to the alrnost 

 constant employment of immersion 

 objectives. And really the only re- 

 searches which are rendered more 

 difficult by the immersion principle 

 are those which have rendered the 

 name of Dr. Dallinger so illustrious. 

 There is no doubt that it is impossible 

 to prevent the admixture of the me- 

 dium with the water below the thin 

 cover when minute organisms are fol- 

 lowed here and there and often close 

 up to the edge of the glass cover. 



Amongst the results of not correct- 

 ing objectives are want of definition, 

 haziness, and the production of certain 

 colors ; and this last phenomenon is 

 often observed in objectives which 



are corrected up to a certain degree 

 and fixed. It is the fashion to correct 

 and fix so as to obtain a certain 

 amount of chromatic aberration, a 

 ruby tint being considered the best. 

 This is to obviate the effect which the 

 perfect achromatism of a glass of large 

 numerical aperture has on the eye. 



There can be no doubt that the 

 majority of the recorded histology of 

 the minuter structures, will have to 

 be worked over again, with carefully 

 corrected objectives. 



In concluding these remarks on the 

 microscope itself, I must enter a pro- 

 test against the clumsy method of 

 pushing a glass slide, with a valuable 

 and important object upon it, with the 

 fingers, under the objective and mov- 

 ing it about. Cheapness of the instru- 

 ment and want of scientific care are 

 the temptations to, and the causes of 

 this very frequent source of error, 

 which is intensified by the common 

 fault of want of perfect fiatness of the 

 plane surface of the stand. 



The beautiful adaptation of a slid- 

 ing glass restricted by a point, whilst 

 it relieves the microscopist from the 

 expenditure involved by a complicated 

 brass movement, is so easily fitted 

 that there is no excuse for employing 

 the fingers alone. 



Instructions in Dry-Plate 

 Photography. 



( Concluded from paL;e log.) 

 To make transparencies for the pro- 

 jecting lantern requires considerable 

 practice, and the exercise of good 

 judgment concerning the printing 

 density of the negative. The begin- 

 ner need not be discouraged if he 

 spoils two dozen jDlates or more in 

 making one dozen good transparen- 

 cies. The loss will be made up in the 

 valuable experience gained. 



'Inhere is no doubt that the best and 

 cheapest transparencies are made with 

 collodion. Nevertheless, it is a fact 

 that excellent transparencies can be 

 made with dry plates, and these are 



