183 



On the Use of Isochromatic Plates in Photomicrography. 



By T. F. Smith, F.R.M.S. 



{Read November 18th, 1892). 



Some months ago a letter appeared in the " English Mechanic" 

 from Mr. Lewis Wright lamenting the smallness of the avail- 

 able supply of f.uorite for the production of apochromatic 

 objectives for the microscope, and further mentioning the diffi- 

 culty of getting it, as a formidable obstacle to future optical 

 improvement. 



With regard to the truth of the charges made in that letter 

 as to the firm of Zeiss having obtained nearly all the fluorite 

 procurable I have nothing to do, but having been engaged for 

 some time past in experimenting with ordinary achromatic 

 lenses as applied to photography, the conclusion I came to was 

 that given certain conditions the total failure of fluorite might 

 not be so disastrous as it seemed. 



My reasons for arriving at this conclusion were, that when 

 using lenses in which the visual and actinic foci were widely 

 divergent when used on ordinary plates, the image came out in 

 sharp focus when isochromatic plates were substituted, thus 

 raising the possessor of an ordinary achromatic objective to the 

 level, photographically, of the owner of the more expensive 

 apochromatic. 



Considering the number of microscopists who now register 

 their results by photographing them, I thought this discovery 

 of mine was so simple a one that I hesitated to announce it as 

 something new, but further inquiries convinced me that it was 

 not so known, and I believe the fact is placed in writing here 

 for the first time. 



Of course, I am aware that the advantage of using isochro- 

 matic plates has been several times mentioned lately, but being 

 always mixed up with the use of a coloured screen or other 



