The President's Address. By E. M. Nelson . 155 
and the apertures have been correspondingly increased. This is a 
promising field, but for the future we must look to the chemist and 
glass manufacturer, rather than to the optician, for further improve- 
ment. Durable glass possessing more favourable optical qualities than 
any as yet catalogued is required for the production of better lenses. 
The semi-apochromat.ic oil 1/12 has had its aperture enlarged, so 
that its Optical Index is now 10 ‘8 against 10' 4. Messrs. Leitz have 
made lately a fine oil 1/10 for the long tube with an O.I. of 13*0. 
Their No. 5 (formerly known as P 7) which is a 1/4 with an O.I. of 
18*6, is a remarkably fine lens. Unfortunately it requires a tube 
only 4J or 5 inches long to obtain correction on a common balsam- 
mounted object with a thickish cover-glass. 
It is high time to speak out against this modern craze for objec- 
tives corrected for very short tubes, such as 100-120 mm. I suppose 
the supply is equal to the demand, and if the truth were known, the 
demand is occasioned by purchasers who have not the very slightest 
idea when their objectives are in or out of correction, and who regard 
and use a Microscope merely as if it were an enlarged magnifying 
glass. Probably the best results are to be obtained when objec- 
tives are corrected for a 10-in. optical tube, which is equivalent to 
about 8f-in. (222 mm.) mechanical tube. 
Coming to more recent times, Messrs. Swift have issued a series 
of semi-apochromats of very fine quality. It is neither the rule nor 
the custom here to enter into the question of the cost of apparatus, 
but the exceptional fall, which every one must have observed, in the 
price of Microscope objectives of high quality, possessing large optical 
indices, has been so remarkable that it cannot be passed over. For 
example, the above-mentioned oil-immersion 1/10 is about the same 
price as a first class 2/3 in. was when I took up Microscopy in 1866. 
Messrs. Swift, who were the pioneers in this country of cheap objec- 
tives which had optical indices worth speaking of, have issued their 
latest series at prices quite on a par with those which obtain on the 
Continent. Nor is this reduction of price confined to semi-apochro- 
mats, for both Messrs. Powell and Swift make fluorite apochromats 
at prices which I believe opticians on the Continent have not yet 
reached. 
Semi-apochromatic condensers also have been greatly improved; 
both Messrs. Beck and Watson make excellent condensers at a price 
hitherto unheard of. With regard to condensers, it must be borne in 
mind that the finest condenser can be reduced to the level of a most 
inferior one unless the slip, the object under examination is mounted 
upon, be of a certain thickness. There has been a fashion among 
mounters for a long time past of putting up selected and test speci- 
mens on “ extra thin ” slips. This was done in order to facilitate the 
employment of a very oblique beam of illumination in one or more 
azimuths. The fallacy underlying this kind of illumination has been 
pointed out ad nauseam. 
