THE AMERICAN MONTHLY 



[January, 



a small scale of an insect, or a diatom, 

 and secure it in position on the stage. 

 Then place the point of the indicator* 

 of the eye-piece upon the object. 

 Every objective shovild show the in- 

 dicator-point upon the object if they 

 all centi'e accurately. The most that 

 can be expected is a reasonable ap- 

 proach to accuracy in this respect. 

 The limit of variation permissible w^e 

 cannot definitely state. 



Thei'e is an opportunity offered 

 here for some microscopist to do a 

 good service for his colaborers. It 

 would be of considerable interest to 

 know what are the variations in the 

 centering of objectives. Those of 

 each maker should be compared 

 among themselves and with those 

 of other makers. The way to do this 

 is to place a micrometer scale in the 

 ocular, and measure the change of 

 position of the object with reference 

 to the end of the indicator. The 

 work requires care and patience, but 

 it is worth doing, and is of special 

 importance in testing the new forms 

 of nose-pieces now coming into use. 



To test the binocular. — This is by 

 no means an easy matter for the nov- 

 ice, since it requires some experience 

 to know when the best appearance of 

 relief is obtained. Nevertheless, a lit- 

 tle practice will enable any person with 

 good eyes to determine whether the 

 images in the two fields blend well 

 together, if these instructions are fol- 

 lowed. 



It is possible to acquire the ability 

 to see the two images independently 

 with the two eyes, and thus compare 

 them side by side, and then cause 

 them to come together, when, if they 

 do not blend, the fault will be at once 

 detected. This, however, is not read- 

 ily done at first trial. To test the 

 blending, therefore, proceed thus : 

 Get as perfect illumination in both 

 tubes as possible. Take a well-de- 

 fined opaque object, such as a shell 

 of a polycystine, and throw a strong 



* An indicator for an eye-piece can be 

 made very quickly in the manner described 

 in the succeeding article. 



light upon it. Focus it carefully, and 

 note if the images in both tubes are 

 equally distinct. Probably it will be 

 found that the right-hand tube gives 

 the brightest, and on the whole the 

 clearest, image, but the difference 

 should be very slight. Now draw 

 out the binocular prism, and, while 

 looking into both tubes, slowly push 

 it in. If the light is properly adjust- 

 ed, which is assured by having a dis- 

 tinct white object on a black ground, 

 the image of the object will soon 

 come into view in the left-hand tube, 

 and by carefully repeating the experi- 

 ment several times it will be a com- 

 paratively easy matter to decide 

 whether the image corresponds ex- 

 actly in position to the one in the 

 other tube. If it does not, one will 

 seem to overlap the other or to be 

 quite independent of it, in which case 

 the binocular is obviously imperfect. 

 Another plan is to select a slide 

 of polycystina or diatoms arranged 

 around a central specimen, which will 

 quite fill the field, or a trifle more. 

 Then examine the margin of both 

 fields, and see if the same forms oc- 

 cupy the same positions in the two 

 fields. Thus, suppose in the right- 

 hand field a specimen just touches the 

 margin in one place. Look at the 

 same form in the other field, and see 

 that it occupies the same position. 

 If not, the arrangement is imperfect. 



A Simple Eye-Piece Indicator. 



The indicator is a very useful, but 

 not very common, attachment to an 

 ocular. When showing objects to 

 friends they are very apt to devote 

 their energies to the admiration of 

 pieces of dirt — just what we do not 

 want them to see — rather than to the 

 beautiful, but perhaps very transpa- 

 rent and less distinctly visible, object. 

 At other times, when there are many 

 objects in one field, it is often quite 

 difficult to designate any particular 

 one. This difficulty is very easily 

 removed by the use of an indicator, 

 the point of which is made to indi- 

 cate the object. 



