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37. Draper, "Process," 220.

38. John T. Towson, "On the proper Focus for the Daguerreotype," Philosophical Magazine 15 (November 1839): 381-85.

On page 383 Towson's article suggested still another reason why Draper's all-flint glass spectacle lens held an advantage over the achromatic lenses Daguerre recommended:

we might imagine that crown glass would be the best material for photographic lenses. This however is not the case. The least dispersive lenses intercept the greatest number of chemical rays, and therefore those of crown glass, and consequently achromatic lenses, cannot be advantageously employed for photographic purposes.

Flint glass evidently passed more light, and therefore made a faster lens. Possibly Draper's work with chromatic lenses of pure flint glass held a significant advantage, in the hands of a skilled operator, over achromatic lenses made of both flint and crown glass. This advantage would have lasted until at least the spring of 1840, when the French began to produce achromatic lenses of large enough size to pass enough light to negate the advantage of flint over crown glass. It was in the spring that Draper himself switched to French achromatics in his portrait camera. Draper, "First Portrait," 5.

Dr. Draper provided still another explanation as to why he was able to work sucessfully in 1839 and 1840 with chromatic lenses. In 1861 Draper chaired a meeting of the American Photographic Society where discussion focused upon a specific advantage of using a chromatic lens of pure flint glass over an achromatic lens. Draper alluded back to his early work with the daguerreotype:

We are apt to think that when lenses are optically perfect they ought be so chemically. This remark was made in 1840, when many excellent daguerreotypes were made, by the use of the ordinary spectacle lens. But at that time our material sensitive to light was iodide of silver only. Iodide of silver is affected very little by other rays than the indigo. The image of the sun from an uncorrected lens on iodide of silver is extremely sharp. When collodian, therefore, is prepared with only an iodide, it is not essential that the camera lens be carefully achromatic. The bromide of silver is, however, decomposed in a larger part of the spectrum, and with it, it is important to work with corrected lenses.

"The American Photographic Society twenty-fourth meeting," The American Journal of Photography, n.s., 3 (1861-62): 270.

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