Journal of Applied Microscopy. 



table, and therefore allow a quicker sum- 

 mation. 



In constructing- such a table, It is of 

 course essential that every variable 

 which can affect the value of a division 

 of the ocular micrometer should be ac- 

 curately noted, e. g., the tube length and 

 the maker's number on the objective; 

 even the particular ocular employed * 

 should be indicated, for the collective 

 lens is not exactly the same for all ocu- 

 lars bearing the same designation. While 

 it is not necessary to express in the table 

 values less than tenths of micra, it is of 

 course necessary to compute the value of 

 one division to hundredths of micra, so 

 that when multiplied, e. g., by nine, the 

 true result may be expressed to the near- 

 est tenth of a micron. The table given is 

 one of several constructed on the same 

 plan for different tube lengths. A table 

 for a single tube length, however, an- 

 swers most purposes, because the tube 

 length can be quickly adjusted to that 

 employed in making out the table. 



E. L. Mark. 



Cambridge, Mass., Dec. 6, 1897. 



*Oc. 2, No. 34, in the above Table. 



The Determination of Supposed 

 Defects in Microscope Objec- 



tives. 



Edward Bausch. 



The methods of making lenses for mi- 

 croscope objectives, although in the main 

 the same as have been used for many 

 years past, are changed in one direction 

 t)y the leading, reputable makers in so 

 far as to eliminate the personal factor 

 of skill in one or another workman. In 

 the process of grinding and polishing 

 lenses this is accomplished by the so- 

 called "color test," and a standard be- 

 ing established by the maker, it is care- 

 fully maintained by one person whose 

 judgment is authoritative. The next 

 steps of centering, cementing, mounting, 

 and adjusting are all carefully con- 

 trolled, and when the preliminary test- 

 ing in the microscope on suitable test 

 objects is reached, any defects which 

 may exist are so apparent as to be easily 

 distinguished by one skilled in the ar't. 

 Defects may consist of: 



(1.) The wrong figure in one of the 

 surfaces, due to a strain in one of the 

 lenses while it is in process of work. 



(2.) To a decentered lens. 



Either of these defects is easily recog- 

 nized, when it is simply necessary to 

 determine in which lens or sysitem it 

 exists and replace it by another; 

 although it may be said in 

 passing that this necessity sel- 

 dom exists. Tlie deviations in 



spherical and chromatic corrections are 

 represented by the proper adjustment 

 of the relative distances of the systems, 

 in which positions they are permanently 

 fixed. The modern mode of clost control 

 and exact tests brings the variation 

 within such narrow limits that it is al- 

 most impossible to find any difference in 

 objectives of the same kind. In making 

 this statement I am well aware of the 

 common belief that the microscope ob- 

 jectives, dependant as they are upon 

 individual will and skill, are therefore 

 variable in their results, but repeat that 

 this belief is well founded only in the 

 productions of former years and at the 

 present time only by the producers of 

 inferior objectives. It is of course ap- 

 parent that the maintenance of a stand- 

 ard lies within the limitations which 

 the maker may prescribe, but in the case 

 of the few leading, reputable firms, I can 

 attest, from personal knowledge, that a 

 sample of one kind of objective is typi- 

 cal for those of its class. "When, there- 

 fore, complaints are occasionally made 

 that objectives possess defects, among 

 which are principally mentioned inferior 

 definition, spherical abeiration, reduced 

 illumination, and short working dis- 

 tance, the question naturally arises in 

 the mind of the optician, whether, if 

 these complaints are warranted, the ob- 

 jectives are used under normal condi- 

 tions — thalt is, corresponding to those 

 under which they were produced. From 

 his knowledge of the capacity of each ob- 

 jective he must assume that they are 

 not, and it is the purpose of this paper 

 to point out the conditions under which 

 the objectives may be used which create 

 the impression of their being defective. 

 These remarks apply equally to the me- 

 dium and high-power objectives, either 

 drj' or immersion. 



Inferior Definition. 



This may be due to: 



(1.) Covers which vary in their thick- 

 ness considerably froin the standard. 



In the case of thick covers spherical 

 over-correction is created, so that the 

 plane of definition is above the plane or 

 outline of the object. In this case the 

 focal distance of the objective must be 

 increased to obtain definition. 



In the case of thin covers, the best 

 definition lies below the plane of the ob- 

 ject and indicates sperical under-correc- 

 tion. The focal distance of the objective 

 must be decre^ased for the best defini- 

 tion, or spherical correction may be made 

 by suitably increasing the tube length. 



The variation in either direction may 

 be so considerable as to make it impos- 

 sible to obtain any definition at all. 



In oil-immersion objectives, the varia- 

 tions in cover glasses have but little 



