1030 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 247. 



A glance at the table and diagram is sufficient to show that there is 

 about as great diversity as possible in the parts included in ' tube-length,' 

 and that the length in millimetres, including these parts, is likewise very 

 diverse. This has, doubtless, come about simply because there was no 

 general standard, and each optician selected for himself a standard. For 

 the sake of those who use the Microscope, it is hoped 

 that a uniform standard may be chosen, or that, at 

 most, but two standards should be decided on by all 

 opticians. These two lengths in millimetres would 

 probably best be 254 mm. for a long or English 

 ' tube-length,' and 160 mm. for the short or Con- 

 tinental ' tube-length.' Furthermore, the same parts 

 of the Microscope should be included in the ' tube- 

 length,' and the parts included should be readily 

 determinable by the youngest student. The parts 

 included by six of the opticians named above, viz. : 

 from the top of the tube (h) where the ocular is 

 inserted, to the lower end (d) where the objective is 

 screwed in, answer this requirement of simplicity. 

 Without urging this as the best possible selection, it 

 will readily be seen that this ' tube-length ' may be 

 easily measured where the ocular and objective are 

 not in position, and that makers of stands vrho do 

 not also make objectives could easily make the tubes 

 of their Microscopes of exactly the right length for 

 the objectives of all objective-makers. While it is 

 ^ true that the objectives of various makers are in 



; mountings of different lengths, and therefore, other 



Diagram showiug the things being equal, tend to increase or diminish the 

 parts of the Micro- actual or optical ' tube-length,' and thus to vary the 

 scope included in magnification of the Microscope, if each maker would 

 'Tube-length' by vari- ghoose the length designated above (&-d) for which 

 world^*'^?See ''table *o correct his objectives in their mountings, then no 

 above.) matter how long or short that mounting might be, 



the microscopist would be able to measure off the 

 right length on the tube of his Microscope, for which the objective was 

 corrected, and having this length once determined, it would not need to be 

 changed when an objective of different length of setting was used. 



Furthermore, the convenience of the microscopist and uniformity in 

 'tube-length' would be both subserved if the eye-pieces or oculars were 

 made ' parfocal,' * that is, the settings be so adjusted that the lower focal 

 points of all the eye-pieces shall be at the same level when in position in 

 the tube of the Microscope, then no refocusing of the Microscope would be 

 necessary upon changing oculars. If also the level of the ' lower focal 

 points ' of the different oculars were made to fall at the level of the top of 

 the body-tube of the Microscope, one end of the so-called ' optical tube- 

 length ' would be always determinable, and correspond with one end, that 

 is the upper end, of the tube of the Microscope. 



So long as no common standard is employed, it seems to the writer 

 that every objective should be accompanied by a statement and a diagram 

 indicating the tube-length in millimetres for which it was corrected, and 

 showing also the parts of the Microscope included in this measurement. 



* See this Journal, 1886, p. 1050. 



