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Partial color blindness among bloodstain examiners.


The Mishnah in tractate Niddah mentions five types of menstrual blood considered tamey (ritually impure):

Red, black, similar to corner of crocus, similar to earth waters, and similar to diluted wine. (Niddah 2:6)

However, after the number of sages expert in this type of examination decreased, the Geonim decreed:

It is not permissible to declare any blood as tahor [ritually pure] if it is reddish. (Saadia Gaon on Sefer Yetsirah, chap. II section 2) For additional sources see Halperin 2009.

The Shulhan Arukh and the Tur rule in accordance with the Geonim:

Every reddish hue is tamey even if it is faint, for we do not know what is considered red and what is considered light or dark. Nonetheless, this is only if the hue inclines to red. (Rabbi Shmuel Ha'Levi Wosner, Shiurei Shevet Ha'Levi, Yoreh Deqh, 188:1, subparagraph 1)

Rabbi Yoel Sirkis explains the rationale for this stringency:

This is because they were concerned they might err in a prohibition that incurs karet (excision) by ruling tamey blood tahor ... Therefore, they made a stringency that any red bloodstain is tamey--even if it does not seem to us as niddah blood ... Thus, we find that Rabbi Yohanan, Rabbi Zeira, and Ula refrained from examining blood to rule whether it was tahor or tamey; they simply ruled that all stains were tamey. Obviously, they did not rule white or green as tamey, for this requires no expertise, as everyone can differentiate between red and white or green. But within red itself, expertise is required [to differentiate] between a tamey red appearance and a tahor red appearance. (Bayit Hadash, Tur Yoreh Deqh 183:12)


Color vision is enabled by the sensitivity of cone cells in the retina to electromagnetic wavelengths. Color differentiation is achieved by pigments in three types of cone cells:

* Red L cones

* Green M cones

* Blue S cones

* Plus overlapping sensitivity regions

The major inherited color vision deficiencies caused by defective or absent cone cells are:

A. Achromacy (total color blindness)

B. Monochromacy (the state of having only one type of functioning cone cells)

C. Dichromacy (the state of having only two types of functioning cone cells)

D. Anomalous trichromacy (all three color cones are present, but the signals are transmitted to the brain abnormally). Types of anomalous trichomacy include:

1. Protanomaly (having a mutated form of the long wavelength [red] pigment). Such people are more likely to be lenient when examining stains.

2. Deuteranomaly (having a mutated form of the medium wavelength [green] pigment). Such people are more likely to be strict in examining stains.

3. Tritanomaly (having a mutated form of the short wavelength [blue] pigment)

The prevalence of red-green color blindness is about 8 percent of males, and about 0.4 percent of females.


The questions that we sought to answer in our research were:

* What is the incidence of partial color blindness (especially anomalous trichromacy) among stain examiners?

* What is the halakhic significance of partial color blindness regarding halakhic decisions on bloodstains?

* Does the ability to decide whether "a certain color inclines towards red" depend solely on the stain examiner's quality of vision?

* How does shimush (apprenticeship with an expert) in examining stains influence partially color-blind apprentices?


The goal

Our research goal was to test changes in the "inclining to see red" tendency of the examinee, correlating to external environmental influence with no change in physiological data.

Five medical tests for partial color blindness were performed on the Round I examinees:

* Ishihara Color Test

* Farnsworth Munsell D-15

* Farnsworth Munsell 100 Hue Test

* Anomaloscope Color Blindness Test

* Red Tendency

Tested population

We tested 76 rabbis undergoing training in stain examination.

First round:

In our first round we tested the degree of "inclining to see red" before the tested candidates commenced their study and apprenticeship. The examinations were conducted under controlled and steady lighting, i.e., indirect sunlight, inside the Schlesinger Institute Building of the Shaare Zedek Medical Center.


Second round:

In our second round we shall test the degree of "inclining to see red" in the candidates after they have completed an intensive course and apprenticeship. Two tests will be used for this stage:

* Anomaloscope

* Red Tendency


The intermediate results of our research revealed:

* There was a verified existence (and variance) of anomalous trichromats (people having a mutated form of their red, green, or blue pigment) within the tested group of rabbinical stain examiners.

* The highest prevalence of color deficiency found was a 2-3 percentage of deuteranomats (people having a pronounced "redness" perception).

* Anomalous trichromats show capacity to differentiate between red hues, in parallel to normal trichromats--in parallel, but not in complete congruence. Both exhibit a capacity to find a specific point of reference in the anomaloscope test (unlike those suffering from dichromacy who cannot settle on a single equilibrium point).

* Intensive apprenticeship can help in identifying anomalous trichromatic cases, but it does not replace quantitative testing.

Qualification Note

Apprenticeship might also have an adverse effect on the differentiation capacity. For example, if a deuteranomalous trichromat apprentices under a normal trichromat, he is liable to learn to disregard what he distortedly recognizes as "red" and thereby skew his recognition process for "real red" stains as well.


We have so far concluded that:

* Monochromats and dichromats (lacking green or red cones) are not qualified to examine stains.

* Anomalous trichromats must be aware of their type of anomaly and its severity:

--Those suffering from deuteranomaly (trichromats who tend to rule stringently) may decide that a stain is tahor without misgivings. In situations where they are uncertain whether to be stringent or not, they should refrain from ruling.

--Those suffering from protanomaly (trichromats who tend to rule leniently) can issue rulings in all situations since the stringency of the Geonim compensates for their visual anomaly. (see Halperin 2009)

* Apprenticeship in stain examination may help in identifying anomalous trichromats, but cannot serve as a substitute for medical diagnosis. Furthermore, there are situations where apprenticeship is liable to cause distortion. For example, if a "stringent" anomalous trichromat apprentices under a normal-sighted stain examiner, he will learn from him to permit stains that to his stringent vision seem to incline towards red. This may lead him to being lenient even with stains that people with normal vision see as inclining to red--which are obviously forbidden.


Halperin, M. 2009. "Partial Color Blindness and Rabbinic Examination of Blood" Jewish Medical Ethics, 7:1, pp. 50-60.

The research was conducted with the support of the Israel Ministry of Science and Technology, 2010-2011.


The Dr. Falk Schlesinger Institute for Medical-Halakhic Research at the Shaare Zedek Medical Center, Jerusalem
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Article Details
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Author:Eliav, Moshe; Belfer, Israel; Halperin, Rabbi Mordechai
Publication:B'Or Ha'Torah
Geographic Code:1USA
Date:Jan 1, 2011
Previous Article:Poster papers.
Next Article:Measuring the Shabbat boundary.

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